More devoted trustee admits regret (5)
I believe the answer is:
truer
'more devoted' is the definition.
(true can mean devoted or loyal)
'trustee admits regret' is the wordplay.
'trustee' becomes 'tr' (abbreviation).
'admits' means one lot of letters goes inside another.
'regret' becomes 'rue' (to rue something is to regret it).
'tr' placed around 'rue' is 'TRUER'.
(Other definitions for truer that I've seen before include "More faithful or straight" , "More relaible" , "More accurate" , "That's more like it" , "More exact" .)