Extremely upset clergyman on holiday finally (4)
I believe the answer is:
very
'extremely' is the definition.
(very can mean extremely or considerably)
'upset clergyman on holiday finally' is the wordplay.
'upset' says the letters should be written backwards.
'clergyman' becomes 'rev' (short for reverend).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'finally' indicates one should take the final letters.
The last letter of 'holiday' is 'y'.
'rev' in reverse letter order is 'ver'.
'ver'+'y'='VERY'
(Other definitions for very that I've seen before include "To a large extent" , "Exactly, really" , "Ever so" , "Extremely - light" , "Kind of light" .)