Gym needed by a climber (3)
I believe the answer is:
pea
'climber' is the definition.
(the pea plant is a climbing plant)
'gym needed by a' is the wordplay.
'gym' becomes 'pe' (Physical Education).
'needed by' says to put letters next to each other.
'pe'+'a'='PEA'
(Other definitions for pea that I've seen before include "Climber" , "Ape this plant badly" , "Little green vegetable" , "Seed eaten as vegetable" , "Leguminous seed" .)