By the way, a very big rugby player is boring (7)
I believe the answer is:
apropos
'by the way' is the definition.
(French-derived term meaning 'by the way')
'a very big rugby player is boring' is the wordplay.
'very big' becomes 'os' (abbreviation for outsize).
'rugby player' becomes 'prop' (type of rugby player).
'is boring' means one lot of letters goes inside another (some letters must 'bore' a hole into other letters).
'a'+'os'='aos'
'aos' going around 'prop' is 'APROPOS'.
(Other definitions for apropos that I've seen before include "Touching" , "To the point, relevant" , "With regard to" , "by the way" , "Suitable or relevant" .)