One caught engrossed by deep fundamental principles (6)
I believe the answer is:
basics
'fundamental principles' is the definition.
'basics' can be an answer for 'principles' (I've seen this before). I'm not sure about the 'fundamental' bit.
'one caught engrossed by deep' is the wordplay.
'one' becomes 'i' (Roman numeral).
'caught' becomes 'c' (cricket abbreviation).
'engrossed by' means one lot of letters goes inside another (I've seen this in other clues).
'deep' becomes 'bass' (synonyms).
'i'+'c'='ic'
'ic' inserted within 'bass' is 'BASICS'.
Can you help me to learn more?
(Other definitions for basics that I've seen before include "Rudiments" , "Essential principles" , "Fundamentals, starting-points" .)