A rugby forward getting unusually large? That’s pertinent (7)
I believe the answer is:
apropos
'pertinent' is the definition.
(similar in meaning)
'a rugby forward getting unusually large?' is the wordplay.
'rugby forward' becomes 'prop' (type of rugby player).
'getting' means one lot of letters go next to another.
'unusually large?' becomes 'OS' (short for over-size).
'a'+'prop'+'os'='APROPOS'
'that's' is the link.
(Other definitions for apropos that I've seen before include "well-chosen" , "In respect of" , "Poor sap (anag) - to the purpose" , "To the point, germane" , "Concerned with" .)