Therapist's regret returning after service (7)
I believe the answer is:
masseur
'therapist's' is the definition.
(masseur is a kind of therapist)
'regret returning after service' is the wordplay.
'regret' becomes 'rue' (synonyms).
'returning' is a reversal indicator.
'after' means one lot of letters go next to another.
'service' becomes 'mass' (I've seen this before).
'rue' backwards is 'eur'.
'eur' put after 'mass' is 'MASSEUR'.
(Other definitions for masseur that I've seen before include "One who kneads and rubs" , "Practitioner of a type of physiotherapy" , "He kneads the body" , "Reliever of muscle pain or tension" , "Sue Mars (anag.)" .)