Felon ultimately with not one regret, going around cell (6)
I believe the answer is:
neuron
'cell' is the definition.
(neuron is a kind of cell)
'felon ultimately with not one regret going around' is the wordplay.
'ultimately' indicates one should take the final letters.
'with' means one lot of letters go next to another.
'not one' becomes 'no' (eg 'no houses' means 'not one house').
'regret' becomes 'rue' (to rue something is to regret it).
'going around' shows that the letters should be reversed in order.
The last letter of 'felon' is 'n'.
'no'+'rue'='norue'
'norue' reversed gives 'euron'.
'n'+'euron'='NEURON'
(Other definitions for neuron that I've seen before include "Conductor" , "Transmitting nerve cell" , "It transmits nerve impulses" , "Impulsive thing" , "Cell that conducts nerve impulses" .)