Fitting a rugby player of exceptional size (7)
I believe the answer is:
apropos
'fitting' is the definition.
(I've seen this before)
'a rugby player of exceptional size' is the wordplay.
'rugby player' becomes 'prop' (type of rugby player).
'of exceptional size' becomes 'OS' (oversize).
'a'+'prop'+'os'='APROPOS'
(Other definitions for apropos that I've seen before include "Concerned with" , "Poor sap (anag) - to the purpose" , "To the point about poor spa" , "suitably" , "Appropriate" .)