Surprisingly taut, she's in buff! (10)
I believe the answer is:
enthusiast
'buff' is the definition.
(I've seen this before)
'surprisingly taut she's in' is the wordplay.
'surprisingly' indicates anagramming the letters.
'taut'+'shes'+'in'='tautshesin'
'tautshesin' anagrammed gives 'ENTHUSIAST'.
(Other definitions for enthusiast that I've seen before include "buff" , "Devotee" , "Fan" , "one not at all cool" , "One deeply interested in a subject" .)