Foolishly sang to horses! (4)
I believe the answer is:
nags
'horses' is the definition.
(nag is a kind of horse)
'foolishly sang' is the wordplay.
'foolishly' is an anagram indicator.
'sang' anagrammed gives 'NAGS'.
'to' acts as a link.
(Other definitions for nags that I've seen before include "Pesters" , "Constantly reproaches and complains" , "Scolds continually" , "Constantly finds fault" , "Worries; horses" .)