Cheerful pit ponies initially found in hay (5)
I believe the answer is:
happy
'cheerful' is the definition.
(associated in meaning)
'pit ponies initially found in hay' is the wordplay.
'initially' suggests taking the first letters.
'found in' is an insertion indicator.
The initial letters of 'pit ponies' is 'pp'.
'pp' going inside 'hay' is 'HAPPY'.
(Other definitions for happy that I've seen before include "One of the Seven Dwarves" , "Cheerful - dwarf" , "Blissful" , "Gleeful" , "Well-pleased" .)