Snort when duty holds lieutenant back (7)
I believe the answer is:
chortle
'snort' is the definition.
(thesaurus)
'duty holds lieutenant back' is the wordplay.
'duty' becomes 'chore' (chore is a kind of duty).
'holds' is an insertion indicator.
'lieutenant' becomes 'lt' (abbreviation for lieutenant).
'back' says the letters should be written backwards.
'lt' back-to-front is 'tl'.
'chore' placed around 'tl' is 'CHORTLE'.
'when' acts as a link.
(Other definitions for chortle that I've seen before include "Noisy gleeful laugh" , "Laugh softly" , "Laugh gleefully" , "Chuckle merrily" , "Chuckle gleefully" .)