A rugby forward’s larger than average, by the way (7)
I believe the answer is:
apropos
'by the way' is the definition.
(French-derived term meaning 'by the way')
'a rugby forward's larger than average' is the wordplay.
'rugby forward' becomes 'prop' (type of rugby player).
'larger than average' becomes 'OS' (short for over-size).
'a'+'prop'+'os'='APROPOS'
(Other definitions for apropos that I've seen before include "In respect of" , "With reference to, concerning" , "Touching" , "fitting" , "well-chosen" .)