One entering offers third of beef to marinade (9)
I believe the answer is:
tenderise
'one' is the definition.
The definition and answer are not the same part of speech.
'entering offers third of beef to marinade' is the wordplay.
'entering' means one lot of letters goes inside another.
'offers' becomes 'tenders' ('tender' can be a synonym of 'offer').
'third of beef' becomes 'e' (3rd letter of 'beef').
'to marinade' becomes 'i' (I can't justify this - if you can you should believe this answer much more).
'tenders'+'e'='tenderse'
'tenderse' placed around 'i' is 'TENDERISE'.
Can you help me to learn more?
(Other definitions for tenderise that I've seen before include "pound" , "soften flesh" .)