Strong point in favour of long hair (8)
I believe the answer is:
fortress
'strong point' is the definition.
(I've seen this before)
'in favour of long hair' is the wordplay.
'in favour of' becomes 'for' (being for a particular cause).
'long hair' becomes 'tress' (I've seen this in another clue).
'for'+'tress'='FORTRESS'
(Other definitions for fortress that I've seen before include "Military stronghold" , "Protective structure" , "Secure building" , "Perhaps keep" , "Castle" .)