In favour of interrupting hair treatment to play a part (7)
I believe the answer is:
perform
'play a part' is the definition.
(thesaurus)
'in favour of interrupting hair treatment' is the wordplay.
'in favour of' becomes 'for' (I've seen this before).
'interrupting' means one lot of letters goes inside another.
'hair treatment' becomes 'perm'.
'for' placed within 'perm' is 'PERFORM'.
'to' acts as a link.
(Other definitions for perform that I've seen before include "Carry out (action)" , "Carry out, accomplish" , "Carry out (duties)" , "Carry out (a task)" , "Do a good job" .)