Batty at heart to protect trees initially (2,5)
I believe the answer is:
at first
'initially' is the definition.
(synonyms)
'batty at heart to protect trees' is the wordplay.
'at heart' means to look at the middle letters.
'to protect' is an insertion indicator (inserted letters are held or protected).
'trees' becomes 'firs' (fir is a kind of tree).
The middle of 'batty' is 'att'.
'att' enclosing 'firs' is 'AT FIRST'.
(Other definitions for at first that I've seen before include "In the beginning, initially" , "In the beginning, to start with" , "To begin with" , "Originally" .)