Policeman in helicopter losing height (6)
I believe the answer is:
copper
'policeman' is the definition.
(copper can informally mean a police officer)
'helicopter losing height' is the wordplay.
'helicopter' becomes 'chopper' (synonyms).
'losing' is a deletion indicator.
'height' becomes 'h' (abbreviation in geometry).
'chopper' with 'h' taken out is 'COPPER'.
'in' acts as a link.
(Other definitions for copper that I've seen before include "Metal, Cu" , "detector?" , "An element; small coin" , "One who nicks" , "Metal - police officer" .)