Stubborn bloke supports strain in exercising (12)
I believe the answer is:
intransigent
'stubborn' is the definition.
(thesaurus)
'bloke supports strain in exercising' is the wordplay.
'bloke' becomes 'gent' (I've seen this in other clues**).
'supports' means one lot of letters go next to another (bottom letters provide a support for the top ones).
'exercising' is an anagram indicator.
'strain'+'in'='strainin'
'strainin' is an anagram of 'intransi'.
'gent' put after 'intransi' is 'INTRANSIGENT'.
(Other definitions for intransigent that I've seen before include "Stubborn" , "Hardliner" , "Obstinate" , "becomes immovable" , "Not prepared to adapt or compromise" .)