Involve oneself enthusiastically with one entering Red Sea port (4,2)
I believe the answer is:
wade in
'involve oneself enthusiastically' is the definition.
(as in wading into a situation)
'with one entering red sea port' is the wordplay.
'with' becomes 'w' (abbreviation).
'one' becomes 'i' (roman numeral).
'entering' means one lot of letters goes inside another.
'red sea port' becomes 'aden' (I've seen this before).
'i' inserted within 'aden' is 'adein'.
'w'+'adein'='WADE IN'
(Other definitions for wade in that I've seen before include "Informally. intervene or make attack - or paddle?" , "Intervene, attack vigorously - in shallows" .)