Assistant interrupted by second digression (5)
I believe the answer is:
aside
'digression' is the definition.
(synonyms)
'assistant interrupted by second' is the wordplay.
'assistant' becomes 'aide' (aide is a kind of assistant).
'interrupted by' indicates putting letters inside.
'second' becomes 's' (common abbreviation).
'aide' placed around 's' is 'ASIDE'.
(Other definitions for aside that I've seen before include "remark, incidentally" , "Remark by an actor, directly to audience say" , "Where one's taken for intimacy" , "Actor's remark for the audience alone" , "Privately, apart" .)