A bit of the harbour charge (7)
I believe the answer is:
portion
'a bit' is the definition.
(thesaurus)
'harbour charge' is the wordplay.
'harbour' becomes 'port' (I've seen this before).
'charge' becomes 'ion' (an ion is a charged particle).
'port'+'ion'='PORTION'
'of the' acts as a link.
(Other definitions for portion that I've seen before include "Share, helping" , "allotment" , "Part - share" , "A lot of" , "Helping - fate" .)