Laugh when officer turns up, interrupting task (7)
I believe the answer is:
chortle
'laugh' is the definition.
(chortling is a kind of laughing)
'officer turns up interrupting task' is the wordplay.
'officer' becomes 'lt' (abbreviation for lieutenant).
'turns up' says the letters should be written backwards.
'interrupting' indicates putting letters inside.
'task' becomes 'chore' (synonyms).
'lt' back-to-front is 'tl'.
'tl' inserted inside 'chore' is 'CHORTLE'.
'when' is the link.
(Other definitions for chortle that I've seen before include "Titter" , "Guffaw" , "Laugh out loud" , "Chuckle merrily" , "Loud gleeful chuckle" .)