Foam covering European potato skin is unwelcome grub (13)
I believe the answer is:
leatherjacket
'unwelcome grub' is the definition.
(grub can mean an insect)
'foam covering european potato skin' is the wordplay.
'foam' becomes 'lather' (lathering is a kind of foaming).
'covering' means one lot of letters goes inside another.
'european' becomes 'e' (abbreviation e.g. EU).
'potato skin' becomes 'jacket' (I've seen this in another clue).
'lather' going around 'e' is 'leather'.
'leather'+'jacket'='LEATHERJACKET'
'is' is the link.
(Other definitions for leatherjacket that I've seen before include "Lava of the crane-fly" , "Cranefly larva" , "Hold close" , "Tropical fish -- heckler at a jet (anag)" , "Garden pest - crane fly larva" .)