I am entering E, say, as rough guess (8)
I believe the answer is:
estimate
'rough guess' is the definition.
(I've seen this before)
'i am entering e say' is the wordplay.
'i am' becomes 'im'.
'entering' is an insertion indicator.
'say' becomes 'state' ('state' can be a synonym of 'say'**).
'e'+'state'='estate'
'im' put inside 'estate' is 'ESTIMATE'.
'as' is the link.
(Other definitions for estimate that I've seen before include "Guess the cost" , "Make approximate judgement of size or amount" , "Form opinion of - tea times (anag)" , "Likely amount" , "Tease Tim (anag.)" .)