Summon heads to come in to explain (4)
I believe the answer is:
cite
'summon' is the definition.
(I know that summon can be written as cite)
'heads to come in to explain' is the wordplay.
'heads to' says to take the initial letters (I've seen 'heading to' mean this (the head of the word)).
The initial letters of 'come in to explain' is 'CITE'.
(Other definitions for cite that I've seen before include "Quote (a reference)" , "Quote (eg, a book)" , "Adduce as proof" , "Quote or refer to" , "Make reference to" .)