Different nail hammered into small instrument (7)
I believe the answer is:
unalike
'different' is the definition.
(I've seen this in another clue)
'nail hammered into small instrument' is the wordplay.
'hammered' is an anagram indicator (slang term for drunk).
'into' means one lot of letters goes inside another.
'small instrument' becomes 'uke' (short for ukulele; 'small' indicates an abbreviation).
'nail' anagrammed gives 'nali'.
'nali' placed inside 'uke' is 'UNALIKE'.
(Other definitions for unalike that I've seen before include "Differing from each other" , "different" , "Incompatible" , "Not similar to one another" , "Dissimilar" .)