Accompanied by genetic material, on reflection (3)
I believe the answer is:
and
'accompanied by' is the definition.
(I've seen this in another clue)
'genetic material on reflection' is the wordplay.
'genetic material' becomes 'DNA' (deoxyribonucleic acid).
'on reflection' shows that the letters should be reversed in order.
'dna' backwards is 'AND'.
(Other definitions for and that I've seen before include "it needs wholly positive input for positive result" , "Most-used conjunction in English" , "Also" , "In addition" , "& 18 You bet!" .)