Altitude without hydrogen for the crew (5)
I believe the answer is:
eight
'the crew' is the definition.
(I've seen this before)
'altitude without hydrogen' is the wordplay.
'altitude' becomes 'height' ('height' can be a synonym of 'altitude').
'without' suggests deleting specific letters.
'hydrogen' becomes 'h' (H is the chemical symbol for hydrogen).
'height' with 'h' taken out is 'EIGHT'.
'for' is the link.
(Other definitions for eight that I've seen before include "Number (with pieces?)" , "2 cubed" , "23" , "Team involved in row" , "Number; rowing boat" .)