Model minces about taking exercise (8)
I believe the answer is:
specimen
'model' is the definition.
(thesaurus)
'minces about taking exercise' is the wordplay.
'about' indicates an anagram.
'taking' means one lot of letters goes inside another.
'exercise' becomes 'pe' (Physical Education).
'minces' with letters rearranged gives 'scimen'.
'scimen' placed around 'pe' is 'SPECIMEN'.
(Other definitions for specimen that I've seen before include "Derogatory description of person" , "Example regarded as typical of its class" , "Men's epic (anag.)" , "Sample to be studied" , "Individual example" .)