Modest, taking small plunge (7)
I believe the answer is:
descent
'plunge' is the definition.
(thesaurus)
'modest taking small' is the wordplay.
'modest' becomes 'decent' (I've seen this before).
'taking' means one lot of letters goes inside another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'decent' enclosing 's' is 'DESCENT'.
(Other definitions for descent that I've seen before include "Slope leading from top" , "Derivation from an ancestor - going down" , "Downward journey" , "Ancestry -- downward movement" , "Ancestry - downward move" .)