Substantial part of molecular genetics? (5)
I believe the answer is:
large
'substantial' is the definition.
(I've seen this before)
'part of molecular genetics?' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'LARGE' can be found hidden inside 'molecular genetics'.
(Other definitions for large that I've seen before include "Sizeable" , "Titanic" , "broad" , "substantial" , "Of great size" .)