Climb a small church? Not without oxygen (6)
I believe the answer is:
ascent
'climb' is the definition.
(I know that climb can be written as ascent)
'a small church? not without oxygen' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'church?' becomes 'CE' (abbreviation for Church of England).
'without' is a deletion indicator.
'oxygen' becomes 'O' (atomic symbol for oxygen).
'not' with 'o' taken away is 'nt'.
'a'+'s'+'ce'+'nt'='ASCENT'
(Other definitions for ascent that I've seen before include "Incline" , "A climb or way up" , "Acclivity" , "advancement" , "rising?" .)