Boundary hidden at first: I’ve found it! (2-2)
I believe the answer is:
ha-ha
'boundary' is the definition.
(type of boundary in a garden)
'hidden at first i've found it' is the wordplay.
'at first' indicates taking the first letters.
'i've found it' becomes 'aha' (exclamation said after finding something).
The initial letter of 'hidden' is 'h'.
'h'+'aha'='HA-HA'
(Other definitions for ha-ha that I've seen before include "Hidden ditch" , "Sunk fence" , "Ditch with wall in it - sounds funny" , "Ditch with wall in it - sounds hilarious" , "Ditch used by landscapers" .)