Person watching play — could be endless pain in neck about it? (6)
I believe the answer is:
critic
'person watching play' is the definition.
'endless pain in neck about it?' is the wordplay.
'endless' means to remove the last letter.
'pain in neck' becomes 'crick'.
'about' means one lot of letters goes inside another.
'crick' with its last letter removed is 'cric'.
'cric' placed around 'it' is 'CRITIC'.
'could be' is the link.
(Other definitions for critic that I've seen before include "One writing reviews" , "Writer of reviews" , "Judge of literary or artistic works" , "Evaluator, reviewer" , "Professional analyst of works of art" .)