Foot beginning to ache? Peasant is put out (8)
I believe the answer is:
anapaest
'foot' is the definition.
(type of metrical foot in poetry)
'beginning to ache? peasant is put out' is the wordplay.
'beginning to' indicates taking the first letters.
'is put out' indicates an anagram.
The initial letter of 'ache' is 'a'.
'peasant' anagrammed gives 'napaest'.
'a'+'napaest'='ANAPAEST'
(Other definitions for anapaest that I've seen before include "part of a line of verse" , "involved in the destruction of Sennacherib?" , "Metrical foot with two syllables followed by one stressed syllable - try a peasant" .)