Unhappy taking part in crusade (3)
I believe the answer is:
sad
'unhappy' is the definition.
('sad' is associated in meaning with 'unhappy')
'part in crusade' is the wordplay.
'part in' indicates the answer is hidden within the clue.
'SAD' is hidden within 'crusade'.
'taking' is the link.
(Other definitions for sad that I've seen before include "upset" , "grey" , "Dejected" , "Unfortunate" , "Disconsolate" .)