Warden's annoyance following onset of rain (6)
I believe the answer is:
ranger
'warden's' is the definition.
(I've seen this before)
'annoyance following onset of rain' is the wordplay.
'annoyance' becomes 'anger' (I've seen this in other clues).
'following' says to put letters next to each other.
'onset of' suggests taking the first letters.
The first letter of 'rain' is 'r'.
'anger' after 'r' is 'RANGER'.
(Other definitions for ranger that I've seen before include "Forest official" , "Forest or park keeper" , "Park employee" , "Forest defender" , "park-keeper" .)