Take part in French joke, eagerly to begin with (6)
I believe the answer is:
engage
'take part' is the definition.
(engage can mean to get involved)
'in french joke eagerly to begin with' is the wordplay.
'in french' becomes 'en' ('in' in French).
'joke' becomes 'gag' (a gag is a joke or witticism).
'to begin with' indicates taking the first letters.
The initial letter of 'eagerly' is 'e'.
'en'+'gag'+'e'='ENGAGE'
(Other definitions for engage that I've seen before include "Secure" , "Take into service or commit to marriage" , "Thief (rhyming slang)" , "attack" , "Employ as a worker" .)