Saw tits periodically circling trees to begin with (2,5)
I believe the answer is:
at first
'begin with' is the definition.
(I've seen this before)
'saw tits periodically circling trees' is the wordplay.
'periodically' means one should take alternating letters (letters must be selected 'every so often').
'circling' indicates putting letters inside (circling can mean going around or containing).
'trees' becomes 'firs' (fir is a kind of tree).
'saw'+'tits'='sawtits'
The alternate letters of 'sawtits' are 'att'.
'att' placed around 'firs' is 'AT FIRST'.
'to' is the link.
(Other definitions for at first that I've seen before include "In the beginning, to start with" , "Originally" , "To begin with" , "In the beginning, initially" .)