Express glee seeing officer caught up in boring duty (7)
I believe the answer is:
chortle
'express glee' is the definition.
(to chortle is to laugh)
'officer caught up in boring duty' is the wordplay.
'officer' becomes 'lt' (abbreviation for lieutenant).
'caught up' says the letters should be written backwards (in a down clue, letters go up).
'in' means one lot of letters goes inside another.
'boring duty' becomes 'chore' (a chore is a boring job).
'lt' reversed gives 'tl'.
'tl' put inside 'chore' is 'CHORTLE'.
'seeing' is the link.
(Other definitions for chortle that I've seen before include "Joyous chuckle" , "Laugh softly" , "Guffaw" , "Loud gleeful chuckle" , "Titter" .)