Fee provided by petty officer at part of journey (7)
I believe the answer is:
'fee' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'petty officer at part of journey' is the wordplay.
'petty officer' becomes 'po' (naval abbreviation).
'at' is a charade indicator (letters next to each other).
'part of journey' becomes 'stage' ('stage' is part of 'journey').
'provided by' is the link.
Can you help me to learn more?
(Other definitions for postage that I've seen before include "Cost to send a mail item", "Amount paid to mail", "Charge for 23 Acrosses", "Cost of corresponding", "Money charged for delivery".)