Sit down in gardens - do the same as teaching assistant earlier (4,1,3)
I believe the answer is:
take a pew
'sit down' is the definition.
(I've seen this before)
'in gardens do the same as teaching assistant earlier' is the wordplay.
'in' is an insertion indicator.
'gardens' becomes 'kew' (Kew Gardens in London).
'do the same as' becomes 'ape' ('ape' can mean 'copy').
'teaching assistant' becomes 'ta' (employee in UK school).
'earlier' is a charade indicator (letters next to each other).
'kew' enclosing 'ape' is 'keapew'.
'keapew' after 'ta' is 'TAKE A PEW'.
(Another definition for take a pew that I've seen is " Please sit!".)