Reasonably decide to reverse charge, one set in stone (3,3)
I believe the answer is:
see fit
'reasonably decide' is the definition.
(to see fit is to deem something acceptable)
'reverse charge one set in stone' is the wordplay.
'reverse' is a reversal indicator.
'charge' becomes 'fee' (fee is a kind of charge).
'one' becomes 'I' (Roman numeral).
'set in' means one lot of letters goes inside another.
'stone' becomes 'st'.
'fee' reversed gives 'eef'.
'eef'+'i'='eefi'
'eefi' put inside 'st' is 'SEE FIT'.
'to' acts as a link.
(Other definitions for see fit that I've seen before include "Consider appropriate, decide to do" , "Consider appropriate or choose to do" , "Consider to be proper or desirable" , "Perceive in good shape, so choose to do?" , "Want" .)