Seen in undergarment? It lessens style (7)
I believe the answer is:
entitle
'style' is the definition.
(thesaurus)
'seen in undergarment? it lessens' is the wordplay.
'seen in' indicates the answer is hidden within the clue.
'ENTITLE' is hidden in the letters of 'undergarment it lessens'.
(Other definitions for entitle that I've seen before include "Name; enable" , "Give (someone) rights" , "Licence" , "Allow" , "Qualify, empower" .)