It's useless to pine foolishly for a perfect ending (5)
I believe the answer is:
inept
'it's useless' is the definition.
(I've seen this before)
'pine foolishly for a perfect ending' is the wordplay.
'foolishly' is an anagram indicator.
'for' means one lot of letters go next to another.
'ending' suggests the final letters (the ending of the word).
The last letter of 'perfect' is 't'.
'pine' is an anagram of 'inep'.
'inep'+'t'='INEPT'
'to' is the link.
(Other definitions for inept that I've seen before include "pointless" , "Lacking skill" , "Showing a lack of competence" , "Unskilful, awkward" , "Foolish" .)