For the climax, a hot number (6)
I believe the answer is:
height
'for the climax' is the definition.
(thesaurus)
'a hot number' is the wordplay.
'a hot' becomes 'h' (abbreviation eg on taps).
'number' becomes 'eight' (eight is a kind of whole number).
'h'+'eight'='HEIGHT'
(Other definitions for height that I've seen before include "Rising ground; extreme example" , "Crowning moment" , "size up?" , "Vertical extent" , "part of a patient's statistics?" .)