A hot number at altitude (6)
I believe the answer is:
height
'altitude' is the definition.
(synonyms)
'a hot number' is the wordplay.
'a hot' becomes 'h' (abbreviation, eg on taps).
'number' becomes 'eight' (eight is a kind of number).
'h'+'eight'='HEIGHT'
'at' is the link.
(Other definitions for height that I've seen before include "Measure it from top to bottom" , "Rising ground; extreme example" , "Distance above ground or sea" , "Stature" , "size up?" .)