Permit enormous current to turn into river (5)
I believe the answer is:
Here is my best explanation:
'permit' is the definition.
'enormous current to turn into river' is the wordplay.
I cannot quite see how this works, but
'turn' could be 't' (abbreviation in Oxford Dictionary of Abbreviations) and 't' is located in the answer.
'river' could be 'r' and 'r' is present in the answer.
The remaining letters 'gan' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
(Other definitions for grant that I've seen before include "Allow; agree to", "Present", "permit", "Give (a right, property, etc)", "Give (a right); gift", "General", "Bursary", "Agree to - bursary", "nominally associated with Ulysses?" and "Greek, a soldier".)