Police team taking top off chain guard (5)
I believe the answer is:
watch
'guard' is the definition.
(I know that guard is a more specific form of the action the watch)
'police team taking top off chain' is the wordplay.
'police team' becomes 'swat' (I've seen this in another clue).
'taking top off' means to remove the first letter.
'chain' becomes 'ch' (abbreviation for the unit of length).
'swat' with its initial letter taken away is 'wat'.
'wat'+'ch'='WATCH'
(Other definitions for watch that I've seen before include "See 21" , "View" , "Timepiece - look out" , "Look at attentively" , "Observe - guard" .)