Physical training keeps pin-up flexible (6)
I believe the answer is:
pliant
'flexible' is the definition.
('pliant' can be a synonym of 'flexible')
'physical training keeps pin-up' is the wordplay.
'physical training' becomes 'pt'.
'keeps' indicates putting letters inside.
'pin' becomes 'nail' (I've seen this before).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'nail' written backwards gives 'lian'.
'pt' going around 'lian' is 'PLIANT'.
(Other definitions for pliant that I've seen before include "Supple or flexible" , "easily modified" , "Malleable, flexible" , "Bendable" , "Flexible - yielding" .)