Tiny pest has power we hear (4)
I believe the answer is:
mite
'tiny pest' is the definition.
(I've seen this in another clue)
'power we hear' is the wordplay.
'power' becomes 'might' (synonyms).
'we hear' indicates a 'sounds like' (homophone) clue.
'might' sounds like 'MITE'.
'has' acts as a link.
(Other definitions for mite that I've seen before include "Little chap" , "Microscopic creature" , "Tiny arachnid, person or amount of money" , "Arachnid; small child" , "Very small creature or child" .)