Begin painting saint overhead (5)
I believe the answer is:
start
'begin' is the definition.
(I know that begin can be written as start)
'painting saint overhead' is the wordplay.
'painting' becomes 'art' (painting is an art).
'saint' becomes 'st' (abbreviation).
'overhead' means one lot of letters go next to another (in a down clue, some letters go over others).
'art' put after 'st' is 'START'.
(Other definitions for start that I've seen before include "Commence" , "begin an argument?" , "launch" , "Origin" , "Commencement" .)