To go on one leg needs hard work (3)
I believe the answer is:
hop
'to go on one leg' is the definition.
I can't judge whether this definition defines the answer.
'hard work' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'work' becomes 'op' (abbreviation for opus).
'h'+'op'='HOP'
'needs' acts as a link.
Can you help me to learn more?
(Other definitions for hop that I've seen before include "Jump about on one leg" , "Beer flavouring; jump" , "Fruit - small jump" , "Dancing party (informal)" , "Jump on a pogo stick" .)