Awkward and formal once but relaxed (13)
I believe the answer is:
uncomfortable
'awkward' is the definition.
(thesaurus)
'formal once but relaxed' is the wordplay.
'relaxed' indicates anagramming the letters.
'formal'+'once'+'but'='formaloncebut'
'formaloncebut' anagrammed gives 'UNCOMFORTABLE'.
'and' acts as a link.
(Other definitions for uncomfortable that I've seen before include "Disquieted" , "Ill at ease" , "Awkward - embarrassed" , "being out of place" , "Awkward - not conducive to relaxing" .)