Spring came, finally producing optimism (4)
I believe the answer is:
hope
'optimism' is the definition.
(thesaurus)
'spring came finally' is the wordplay.
'spring' becomes 'hop' (I've seen this before**).
'finally' says to take the final letters.
The final letter of 'came' is 'e'.
'hop'+'e'='HOPE'
'producing' is the link.
(Other definitions for hope that I've seen before include "Feeling of expectation or desire" , "'. . . . springs eternal in the human breast'" , "English-born US comedian, d. 2003" , "Expect and desire" , "virtue he needs?" .)