Meticulous firm promises to hide identity (10)
I believe the answer is:
fastidious
'meticulous' is the definition.
(similar in meaning)
'firm promises to hide identity' is the wordplay.
'firm' becomes 'fast' (synonyms).
'promises' becomes 'ious' (IOU's, promises to pay a debt).
'to hide' is an insertion indicator.
'identity' becomes 'id' (I've seen this in other clues).
'fast'+'ious'='fastious'
'fastious' enclosing 'id' is 'FASTIDIOUS'.
(Other definitions for fastidious that I've seen before include "Attentive to detail" , "Meticulous" .)