Police officer’s helicopter losing height (6)
I believe the answer is:
copper
'police officer's' is the definition.
(copper can informally mean a police officer)
'helicopter losing height' is the wordplay.
'helicopter' becomes 'chopper' (synonyms).
'losing' suggests deleting specific letters.
'height' becomes 'h' (abbreviation in geometry).
'chopper' with 'h' taken away is 'COPPER'.
(Other definitions for copper that I've seen before include "Metallic element and British bobby" , "Metal for slang policeman" , "A little cash" , "Low-value coin" , "Little money" .)