Cover its head in toys, spreading firmness (8)
I believe the answer is:
solidity
'firmness' is the definition.
(I've seen this before)
'cover its head in toys spreading' is the wordplay.
'cover' becomes 'lid' (lid is a kind of cover).
'head' indicates taking the first letters.
'in' is an insertion indicator.
'spreading' indicates an anagram.
The first letter of 'its' is 'i'.
'lid'+'i'='lidi'
'toys' anagrammed gives 'soty'.
'lidi' placed into 'soty' is 'SOLIDITY'.
(Other definitions for solidity that I've seen before include "Firmness" , "Substance" , "Firm quality" .)