Felon ultimately with not a single regret turned up in cell (6)
I believe the answer is:
neuron
'cell' is the definition.
(I know that neuron is a type of somatic cell)
'felon ultimately with not a single regret turned up' is the wordplay.
'ultimately' suggests the final letters.
'with' means one lot of letters go next to another.
'not a single' becomes 'no' (in a phrase like 'no man is an island').
'regret' becomes 'rue' (to rue something is to regret it).
'turned up' shows that the letters should be reversed in order.
The final letter of 'felon' is 'n'.
'no'+'rue'='norue'
'norue' reversed gives 'euron'.
'n'+'euron'='NEURON'
'in' is the link.
(Other definitions for neuron that I've seen before include "Part of one's nervous system" , "Impulsive thing" , "Cell that conducts nerve impulses" , "Conductor" , "that makes for nervous transmission" .)