Approach rugby union joke the wrong way (3-2)
I believe the answer is:
run up
'approach' is the definition.
(I know that run-up is a type of approach)
'rugby union joke the wrong way' is the wordplay.
'rugby union' becomes 'RU' (abbreviation).
'joke' becomes 'pun' (pun is a type of joke).
'the wrong way' shows that the letters should be reversed in order.
'pun' written backwards gives 'nup'.
'ru'+'nup'='RUN-UP'
(Other definitions for run up that I've seen before include "To make or mend hastily" , "Quickly make, appear" , "time for preparation" , "formed quickly" , "let bill accumulate" .)