Initially up that tree, pals etc playing (2,3,5,5)
I believe the answer is:
in the first place
'initially' is the definition.
(I've seen this before)
'up that tree pals etc playing' is the wordplay.
'up that tree' becomes 'in the fir'.
'playing' indicates an anagram.
'pals'+'etc'='palsetc'
'palsetc' anagrammed gives 'stplace'.
'inthefir'+'stplace'='IN THE FIRST PLACE'
(Other definitions for in the first place that I've seen before include "Where Adam and Eve lived" , "Where the winner would be" , "originally" , "Where Adam and Eve sinned" , "to begin with" .)