I turn on the foremost of ecclesiastics (3)
I believe the answer is:
ego
'i' is the definition.
(thesaurus)
'turn on the foremost of ecclesiastics' is the wordplay.
'turn' shows that the letters should be reversed in order.
'on' becomes 'og' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'the foremost of' indicates taking the first letters.
The initial letter of 'ecclesiastics' is 'e'.
'og'+'e'='oge'
'oge' written backwards gives 'EGO'.
Can you help me to learn more?
(Other definitions for ego that I've seen before include "Inner self" , "Morale" , "Conscious mind" , "Part of the mind" , "(Person's) sense of self-esteem" .)