A mouth lifted by itself (5)
I believe the answer is:
apart
'by itself' is the definition.
(thesaurus)
'a mouth lifted' is the wordplay.
'mouth' becomes 'trap' (informal term for mouth).
'lifted' is a reversal indicator (going up in down clue).
'trap' in reverse letter order is 'part'.
'a'+'part'='APART'
(Other definitions for apart that I've seen before include "out on a limb?" , "Separated physically" , "Separate in a trap" , "No longer living together" , "(Break) into pieces" .)