Assume I am up in this field of knowledge (6)
I believe the answer is:
domain
'field of knowledge' is the definition.
'domain' can be an answer for 'field' (thesaurus). I am not sure about the 'of knowledge' bit.
'assume i am up in this' is the wordplay.
'assume' becomes 'don' (I've seen this before).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'in this' is an insertion indicator.
'i'+'am'='iam'
'iam' backwards is 'mai'.
'don' placed around 'mai' is 'DOMAIN'.
Can you help me to learn more?
(Other definitions for domain that I've seen before include "estate" , "Scope of knowledge - owned land" , "Range of influence or field of thought" , "Realm or range of influence" , "Field of thought or activity" .)