Keeping a sort of light twinkling all around cell (4)
I believe the answer is:
ovum
'cell' is the definition.
(an ovum is an egg cell)
'keeping a sort of light twinkling all around' is the wordplay.
'keeping' is an insertion indicator.
'a sort of light' becomes 'uv' (short for ultraviolet light).
'twinkling' becomes 'mo' (I've seen this in other clues).
'all around' says the letters should be written in reverse.
'uv' placed within 'mo' is 'muvo'.
'muvo' written backwards gives 'OVUM'.
(Other definitions for ovum that I've seen before include "Human egg cell" , "Reproductive cell, 'egg' in Latin" , "A female egg-cell" , "Female reproductive cell" , "bit of life potentially" .)