Put forward idea initially in correspondence (5)
I believe the answer is:
posit
'put forward' is the definition.
(to posit an idea is to put it forward)
'idea initially in correspondence' is the wordplay.
'initially' says to take the initial letters.
'in' means one lot of letters goes inside another.
'correspondence' becomes 'post' (letters in the post).
The initial letter of 'idea' is 'i'.
'i' placed within 'post' is 'POSIT'.
(Other definitions for posit that I've seen before include "Suggest" , "Put forward; place" , "Assume as a fact or principle" , "Propose" , "Assume as fact, postulate" .)