Career in rugby's hard (4)
I believe the answer is:
rush
'career' is the definition.
(I've seen this in other clues)
'rugby's hard' is the wordplay.
'rugby's' becomes 'ru's'.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'rus'+'h'='RUSH'
'in' acts as a link.
(Other definitions for rush that I've seen before include "Spate" , "'Hurry, haste (4)'" , "Perform hurriedly; marsh plant" , "Stampede - water plant" , "Drug-induced euphoria" .)