Dad's outside initially mowing the grass (6)
I believe the answer is:
pampas
'grass' is the definition.
(I've seen this before)
'dad's outside initially mowing' is the wordplay.
'dad's' becomes 'papa's' ('papa' can be a synonym of 'dad').
'outside' is an insertion indicator.
'initially' says to take the initial letters.
The initial letter of 'mowing' is 'm'.
'papas' going around 'm' is 'PAMPAS'.
'the' is the link.
(Other definitions for pampas that I've seen before include "Great plains of Argentina" , "Plains with grass" , "South American plain" , "Argentina grassy plain" , "Grassy plains of South America" .)