Record is mounted European officer's letter (7)
I believe the answer is:
epistle
'letter' is the definition.
(epistle is a kind of letter)
'record is mounted european officer's' is the wordplay.
'record' becomes 'ep'.
'mounted' shows that the letters should be reversed in order.
'european' becomes 'e' (abbreviation e.g. EU).
'officer' becomes 'lt' (I've seen this in other clues**).
'e'+'lt'='elt'
'elt' in reverse letter order is 'tle'.
'ep'+'is'+'tle'='EPISTLE'
(Other definitions for epistle that I've seen before include "may read this" , "A letter, biblical perhaps" , "may be written off" , "(E.g. Paul's) letter" , "Long formal letter (found in the New Testament?)" .)