Detachment of troops mostly came back abnormally thin (9)
I believe the answer is:
emaciated
'abnormally thin' is the definition.
(emaciated means very thin)
'detachment of troops mostly came back' is the wordplay.
'detachment of troops' becomes 'detail' (detail can mean a small group of soldiers).
'mostly' means to remove the last letter.
'back' says the letters should be written backwards.
'detail' with its final letter taken away is 'detai'.
'detai'+'came'='detaicame'
'detaicame' back-to-front is 'EMACIATED'.
(Other definitions for emaciated that I've seen before include "Extremely thin" , "undernourished" , "of very little substance" , "Cadaverous" , "Abnormally thin and weak" .)