Military helicopter leaves hospital very quickly (5)
I believe the answer is:
apace
'very quickly' is the definition.
'military helicopter leaves hospital' is the wordplay.
'military helicopter' becomes 'apache' (type of US military helicopter).
'leaves' is a deletion indicator.
'hospital' becomes 'h'.
'apache' with 'h' taken out is 'APACE'.
(Other definitions for apace that I've seen before include "(Move on) swiftly" , "at high speed" , "flat out" , "fast" , "Quick" .)