Wingspan 7 to 9 mm. The larva mines the leaves of Alder, creating a tentiform mine between two veins on the underside, with a winkled surface withut sharp folds.  The larva is white or yellow, and the pupa is in a cocoon with frass heaped to one side.

It can be similar to mines of P rajella or P froehlichiella.  If in doubt, examine larva or pupa. (ref. Bladmineerders.nl) 

Adult Leafmine

Photogrpah the under-surface of the mine, and larva/pupa.  Note host species

Around Alder.

The adults fly mostly in August, but the species may produce two broods a year, with moths also recorded in May.

The larva feeds on alder, forming a small blotch mine on the underside of a leaf. Sometimes several larvae mine the same leaf.

Distributed throughout mainland Britain, though rare in the southern half of Scotland and in Ireland, it can be common in places. In the Butterfly Conservation’s Microlepidoptera Report 2011 this species was classified as common.

Once considered rare in Leicestershire and Rutland, but with more leafmining activity taking place in recent years this species has proved to be much more common than previously thought.

http://www.leafmines.co.uk/html/Lepidoptera/P.kleemannella.htm

https://bladmineerders.nl/parasites/animalia/arthropoda/insecta/lepidoptera/ditrysia/gracillarioidea/gracillariidae/lithocolletinae/phyllonorycter/phyllonorycter-klemannella/