A theologian will put more in (3)
I believe the answer is:
add
'put more in' is the definition.
(I've seen this in another clue)
'a theologian' is the wordplay.
'theologian' becomes 'DD' (Doctor of Divinity degree).
'a'+'dd'='ADD'
'will' is the link.
(Other definitions for add that I've seen before include "Put together - append" , "Reckon" , "Augment" , "Go further" , "do some mathematics" .)