Passionate about rest held back (9)
I believe the answer is:
hesitated
'held back' is the definition.
(I've seen this before)
'passionate about rest' is the wordplay.
'passionate' becomes 'heated' (similar in meaning).
'about' indicates putting letters inside.
'rest' becomes 'sit' (I've seen this before).
'heated' enclosing 'sit' is 'HESITATED'.
(Other definitions for hesitated that I've seen before include "Paused, wavered" , "held back" , "Was reluctant" , "Thought twice" , "Was not keen" .)