Jack was obliged to limit one fervent crusade (5)
I believe the answer is:
jihad
'fervent crusade' is the definition.
'jack was obliged to limit one' is the wordplay.
'jack' becomes 'j' (playing card abbreviation).
'was obliged' becomes 'had' (to have to do something is to be obliged to do it).
'to limit' indicates putting letters inside.
'one' becomes 'I' (Roman numeral).
'j'+'had'='jhad'
'jhad' placed around 'i' is 'JIHAD'.
(Other definitions for jihad that I've seen before include "blessed war" , "Holy war as declared by osama Bin Laden" , "Muslim war against unbelievers" , "crusade" , "Holy war in defence of Islamic faith" .)