Weak model beefed up secretly (6)
I believe the answer is:
feeble
'weak' is the definition.
(I know that weak can be written as feeble)
'model beefed up secretly' is the wordplay.
'secretly' says the answer is hidden in the clue.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'elbeef' is hidden in the letters of 'model beefed'.
'elbeef' reversed gives 'FEEBLE'.
(Other definitions for feeble that I've seen before include "Weak, debilitated" , "Lacking force" , "Lacking vitality, very weak" , "Lacking physical strength" , "rather weak" .)