Picked up key for pretty small hole (6)
I believe the answer is:
eyelet
'pretty small hole' is the definition.
(an eyelet is a small hole)
'picked up key' is the wordplay.
'picked up' indicates a 'sounds like' (homophone) clue.
'key' becomes 'islet' (both can mean a small island).
'islet' sounds like 'EYELET'.
'for' acts as a link.
(Other definitions for eyelet that I've seen before include "Small reinforced hole" , "A lace is passed through it" , "Hole in needle through which thread passes" , "hole in shoe leather?" , "Small hole through which a cord is passed" .)