Time to enter cell — it's free of charge (7)
I believe the answer is:
neutron
'it's free of charge' is the definition.
(I've seen this in another clue)
'time to enter cell' is the wordplay.
I cannot quite see how this works, but
'time' could be 't' (abbreviation) and 't' is found in the answer.
'cell' could be 'neuron' (neuron is a kind of cell) and 'neuron' is found in the remaining letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for neutron that I've seen before include "Uncharged subatomic particle" , "Charged particle" , "On tuner (anag.)" , "Elementary particle with zero charge" , "Particle that decays when released" .)