A rugby forward's very large, by the way (7)
I believe the answer is:
apropos
'by the way' is the definition.
(I've seen this before)
'a rugby forward's very large' is the wordplay.
'rugby forward' becomes 'prop' (I've seen this before).
'very large' becomes 'os' (abbreviation for oversize).
'a'+'prop'+'os'='APROPOS'
(Other definitions for apropos that I've seen before include "With regard to" , "Suitable; to the point" , "suitably" , "Concerned with" , "fitting" .)