It's apt to be very soft in the hay (5)
I believe the answer is:
happy
'it's apt' is the definition.
(thesaurus)
'very soft in the hay' is the wordplay.
'very soft' becomes 'pp' (musical abbreviation for pianissimo).
'in the' is an insertion indicator.
'pp' inserted into 'hay' is 'HAPPY'.
'to be' is the link.
(Other definitions for happy that I've seen before include "Cheerful - dwarf" , "Delighted" , "Pleased; fortunate" , "A not so low" , "Fortunate, convenient" .)