Roy's header really annoys 3 (7)
I believe the answer is:
rangers
'roy's header really annoys 3' is the definition.
I can't tell whether this defines the answer.
'roy's header really annoys' is the wordplay.
'header' indicates taking the first letters (in a down clue, the letter at the top of the word).
'really annoys' becomes 'angers' (to anger is to annoy).
The initial letter of 'roys' is 'r'.
'r'+'angers'='RANGERS'
(Other definitions for rangers that I've seen before include "Park/forest keepers" , "Glasgow football club" , "Park officials - Glasgow FC" , "Long-distance travellers" , "Glasgow soccer team" .)