Standard form of gene mostly for twin (8)
I believe the answer is:
parallel
'for twin' is the definition.
(synonyms)
'standard form of gene mostly' is the wordplay.
'standard' becomes 'par' (standard of play in golf).
'form of gene' becomes 'allele' (genetic variation).
'mostly' means to remove the last letter (most but not all of the word).
'allele' with its last letter taken off is 'allel'.
'par'+'allel'='PARALLEL'
(Other definitions for parallel that I've seen before include "Similarity, comparison" , "Imaginary line round the Earth" , "suggesting inability to make projected meeting" , "Everywhere is equidistant and doesn't intersect" , "Of lines, continually at same distance from each other" .)