For a semidetached, it shows potential (7)
I believe the answer is:
'potential' is the definition.
(I've seen this in another clue)
'for a semidetached it' is the wordplay.
'for' becomes 'pro' (as in being 'pro' something).
'a semidetached' becomes 'mis' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'it' becomes 'e' ('e' can mean 'electronic' which is similar to 'IT').
'shows' is the link.
Can you help me to learn more?
(Other definitions for promise that I've seen before include "Pledge - my word!" , "Verbal or other commitment of one person to another" , "Betroth" , "Guarantee" , "Give one's word" .)