By contrast with, I repeat, the last half (7)
I believe the answer is:
against
'half' is the definition.
I know nothing about this answer so I can't tell whether this works.
'by contrast with i repeat the last' is the wordplay.
'by contrast' becomes 'gains' (I can't explain this - if you can you should believe this answer much more).
'with' means one lot of letters go next to another.
'i' becomes 'a'.
'the last' indicates one should take the final letters.
The final letter of 'repeat' is 't'.
'gains' after 'a' is 'agains'.
'agains'+'t'='AGAINST'
Can you help me to learn more?
(Other definitions for against that I've seen before include "Not favouring" , "Placed in direct contact with" , "In disagreement with" , "Abutting" , "In opposition to" .)