Climber in training area (3)
I believe the answer is:
pea
'climber' is the definition.
(I've seen this before)
'training area' is the wordplay.
'training' becomes 'PE' (Physical Education).
'area' becomes 'a' (maths abbreviation).
'pe'+'a'='PEA'
'in' is the link.
(Other definitions for pea that I've seen before include "It may climb frame" , "Little green vegetable" , "Annual climbing plant with edible seeds" , "It grows in 23 Down" , "Spherical vegetable" .)