Helicopter's top crew's altitude (6)
I believe the answer is:
height
'altitude' is the definition.
('height' can be a synonym of 'altitude')
'helicopter's top crew's' is the wordplay.
'top' says to take the initial letters.
'crew' becomes 'eight' (I've seen this before).
The first letter of 'helicopters' is 'h'.
'h'+'eight'='HEIGHT'
(Other definitions for height that I've seen before include "Vertical dimension" , "Stature" , "Vertical extent" , "Peak" , "Measurement from top to bottom" .)