I require a day to go through it, certainly (6)
I believe the answer is:
indeed
'certainly' is the definition.
(both can be used to answer in the affirmative)
'i require a day to go through' is the wordplay.
'require' becomes 'need' (synonyms).
'a day' becomes 'd' (abbreviation for day).
'to go through' indicates putting letters inside.
'need' going around 'd' is 'ndeed'.
'i'+'ndeed'='INDEED'
'it' is the link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for indeed that I've seen before include "Really, without doubt" , "Truly or admittedly" , "Certainly, quite so" , "Unquestionably" , "even" .)