Crew needing altitude without hydrogen (5)
I believe the answer is:
eight
'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 away is 'EIGHT'.
'needing' acts as a link.
(Other definitions for eight that I've seen before include "Two cubed" , "Number etched by a skater?" , "Cube of two" , "Where arrogance is needed" , "Cube root of five hundred and twelve" .)