A pest can be simply too much! (5)
I believe the answer is:
spate
'simply too much' is the definition.
'spate' can be an answer for 'much' (thesaurus). I am not certain of the 'simply too' bit.
'a pest can be' is the wordplay.
'can be' indicates anagramming the letters.
'a'+'pest'='apest'
'apest' with letters rearranged gives 'SPATE'.
Can you help me to learn more?
(Other definitions for spate that I've seen before include "Suddenly got a lot" , "Tapes found in sudden flood" , "Rush, flood (of river)" , "A torrent of tapes" , "rash" .)