Agree with everything in terms of current theory (5)
I believe the answer is:
tally
'agree with' is the definition.
(thesaurus)
'everything in terms of current theory' is the wordplay.
'everything' becomes 'all' (all things).
'in' means one lot of letters goes inside another.
'terms of' suggests the final letters (term can mean the end of something).
The final letters of 'current theory' are 'ty'.
'all' going into 'ty' is 'TALLY'.
(Other definitions for tally that I've seen before include "Coincide" , "Score; correspond" , "Current score; agree" , "Count, add up" , "Count up; match" .)