A foot to cheer wildly (7)
I believe the answer is:
trochee
'a foot' is the definition.
(I know that trochee is a type of foot)
'to cheer wildly' is the wordplay.
'wildly' is an anagram indicator.
'to'+'cheer'='tocheer'
'tocheer' anagrammed gives 'TROCHEE'.
(Other definitions for trochee that I've seen before include "To cheer about this metrical unit" , "Long-short foot (metre)" , "Metric foot; to cheer (anag.)" , "Long-short metric foot" , "poetic feature" .)