Overcome, escort one who is leaving (3,3)
I believe the answer is:
see off
'overcome escort' is the definition.
The definition suggests a verb in its base form which matches the answer.
'one who is leaving' is the wordplay.
'one who' becomes 'see' (I can't justify this - if you can you should believe this answer much more).
'is leaving' becomes 'off' ('I'm off' means 'I'm leaving').
'see'+'off'='SEE OFF'
Can you help me to learn more?
(Other definitions for see off that I've seen before include "Accompany to departure point, or defeat" , "Get rid of (someone)" , "reprimand" , "Repel (an intruder)" , "Accompany to point of departure, or chase away intruder" .)