Mostly frigid and unusually unbecoming (5,3)
I believe the answer is:
infra dig
'unbecoming' is the definition.
(thesaurus)
'mostly frigid and unusually' is the wordplay.
'mostly' means to remove the last letter.
'unusually' is an anagram indicator.
'frigid'+'and'='frigidand'
'frigidand' with its final letter removed is 'frigidan'.
'frigidan' with letters rearranged gives 'INFRA DIG'.
(Other definitions for infra dig that I've seen before include "Beneath oneself (from Latin)" , "Unworthy" , "Beneath one" , "degrading" , "Rigid fan may be lacking in dignity" .)