Modelling work comes first for rival (8)
I believe the answer is:
opposing
'for rival' is the definition.
(thesaurus)
'modelling work comes first' is the wordplay.
'modelling' becomes 'posing' ('pose' can be a synonym of 'model').
'work' becomes 'op' (abbreviation for opus).
'comes first' means one lot of letters go next to another.
'posing' put after 'op' is 'OPPOSING'.
(Other definitions for opposing that I've seen before include "Contending" , "Going against" , "modelling after work" , "trying to end a sea of troubles by it?" , "Disapproving" .)