In poor condition, one accepting frailty at first (5)
I believe the answer is:
unfit
'in poor condition' is the definition.
(I've seen this before)
'one accepting frailty at first' is the wordplay.
'one' becomes 'unit' ('unit' can be a synonym of 'one').
'accepting' indicates putting letters inside.
'at first' indicates taking the first letters.
The first letter of 'frailty' is 'f'.
'unit' going around 'f' is 'UNFIT'.
(Other definitions for unfit that I've seen before include "Not in good physical shape (5)" , "Ill-equipped" , "Not prepared" , "In poor condition" , "Incapable" .)