Assistant keeping son out of the way (5)
I believe the answer is:
aside
'out of the way' is the definition.
('aside' can be a synonym of 'out of the way')
'assistant keeping son' is the wordplay.
'assistant' becomes 'aide' (aide is a kind of assistant).
'keeping' means one lot of letters goes inside another.
'son' becomes 's'.
'aide' placed around 's' is 'ASIDE'.
(Other definitions for aside that I've seen before include "Stage whisper" , "Separately" , "remark, incidentally" , "Where one's taken for intimacy" , "Ideas (anag)" .)