Held back in reserve, and groomed (10)
I believe the answer is:
restrained
'held back' is the definition.
('restrain' can be a synonym of 'hold back')
'reserve and groomed' is the wordplay.
'reserve' becomes 'res' (I've seen this in other clues).
'and' means one lot of letters go next to another.
'groomed' becomes 'trained' ('train' can be a synonym of 'groom').
'res'+'trained'='RESTRAINED'
'in' is the link.
(Other definitions for restrained that I've seen before include "Deprived of freedom" , "subdued" , "Reserved, dispassionate" , "muted" , "Calm" .)