Hardest to find in a fine street (6)
I believe the answer is:
rarest
'hardest to find' is the definition.
(I've seen this in another clue)
'fine street' is the wordplay.
'fine' becomes 'rare' (I've seen this in another clue).
'street' becomes 'st' (abbreviation).
'rare'+'st'='RAREST'
'in a' is the link.
(Other definitions for rarest that I've seen before include "fine to maximum" , "Most scarce and probably precious" , "least done" , "most excellent" , "Most scarce or uncommon" .)