Restrained but losing head in hurry (5)
I believe the answer is:
haste
'hurry' is the definition.
(haste is hurrying or rushing)
'restrained but losing head' is the wordplay.
'restrained' becomes 'chaste' (chaste can mean restrained or simple).
'but losing head' means to remove the first letter.
'chaste' with its initial letter taken away is 'HASTE'.
'in' acts as a link.
(Other definitions for haste that I've seen before include "Expeditiousness" , "velocity" , "Hurriedness" , "Inconsiderate speed" , "Rash speed" .)