Becoming a rugby player, very big (7)
I believe the answer is:
apropos
'becoming' is the definition.
(thesaurus)
'a rugby player very big' is the wordplay.
'rugby player' becomes 'prop' (I've seen this in other clues).
'very big' becomes 'os' (abbreviation for outsize).
'a'+'prop'+'os'='APROPOS'
(Other definitions for apropos that I've seen before include "Appropriate" , "Suitable; to the point" , "Touching" , "Poor sap, being to the point" , "To the point, relevant" .)