Keenly, say, about to come in before due time (7)
I believe the answer is:
eagerly
'keenly' is the definition.
(I've seen this before)
'say about to come in before due time' is the wordplay.
'say' becomes 'eg' (both can mean for example).
'about' says the letters should be written in reverse.
'to come in' means one lot of letters goes inside another.
'before due time' becomes 'early' (I've seen this before).
'eg' back-to-front is 'ge'.
'ge' inserted into 'early' is 'EAGERLY'.
(Other definitions for eagerly that I've seen before include "Enthusiastically" , "anxiously" , "Fervently" , "Keenly" , "Showing impatience" .)