Face pain, showing self-assurance (7)
I believe the answer is:
panache
'self-assurance' is the definition.
(thesaurus)
'face pain' is the wordplay.
'face' becomes 'pan' ('pan' is old-fashioned US slang for 'face').
'pain' becomes 'ache' (ache is a kind of pain).
'pan'+'ache'='PANACHE'
'showing' acts as a link.
(Other definitions for panache that I've seen before include "Dashing style" , "Dash and verve" , "A confident stylish manner" , "flourish" , "Flamboyant sense of style" .)