Assistant keeps son out of the way (5)
I believe the answer is:
aside
'out of the way' is the definition.
(synonyms)
'assistant keeps son' is the wordplay.
'assistant' becomes 'aide' (aide is a kind of assistant).
'keeps' is an insertion indicator.
'son' becomes 's'.
'aide' enclosing 's' is 'ASIDE'.
(Other definitions for aside that I've seen before include "In reserve; actor's remark to audience" , "Line spoken to theatre audience" , "Aides (anag.)" , "Line spoken to the audience of a play" , "secretly" .)