Implacable, passed on loan with deductions (10)
I believe the answer is:
relentless
'implacable' is the definition.
(similar in meaning)
'passed on loan with deductions' is the wordplay.
I cannot quite understand how this works, but
'on' could be 're' ('on' can be mean 'with reference to') and 're' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for relentless that I've seen before include "Allowing no respite" , "Uncompromising" , "Oppressively constant; inflexible" , "Inexorable" , "Persistent" .)