Fellow captured by detachment is in bad shape (5)
I believe the answer is:
unfit
'bad shape' is the definition.
'unfit' can be an answer for 'bad' (thesaurus). I'm not sure about the 'shape' bit.
'fellow captured by detachment' is the wordplay.
'fellow' becomes 'f' (abbreviation - of a society etc.).
'captured by' means one lot of letters goes inside another.
'detachment' becomes 'unit' (I've seen this before).
'f' put into 'unit' is 'UNFIT'.
'is in' acts as a link.
Can you help me to learn more?
(Other definitions for unfit that I've seen before include "Unsuitable" , "Not in good shape physically" , "Ill - out of shape" , "Incapable" , "Incompetent" .)