Massive job mujaheddin must keep up (5)
I believe the answer is:
jumbo
'massive' is the definition.
(synonyms)
'job mujaheddin must keep up' is the wordplay.
'keep' indicates a hidden word.
'up' is a reversal indicator (in down clue: letters go upwards).
'obmuj' is hidden within 'job mujaheddin must'.
'obmuj' written backwards gives 'JUMBO'.
(Other definitions for jumbo that I've seen before include "Immense" , "Huge (plane or elephant?)" , "Unusually large" , "monster" , "Elephant(ine)" .)