Officer catching 'Sarge' heading down is the greatest (7)
I believe the answer is:
largest
'the greatest' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for largest that I've seen before include "Extremely generous" , "most great" , "Chief" , "Most substantial" , "Of greatest magnitude" .)