Jack's a big disappointment (5)
I believe the answer is:
frost
'jack's' is the definition.
The answer and definition can be both to do with contact as well as being verbs in their base form.
Maybe there's a link between them I don't understand?
'a big disappointment' is the wordplay.
I cannot quite see how this works, but
'big' could be 'os' and 'os' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for frost that I've seen before include "Coldness of manner" , "Icy condensation" , "Hoar" , "Rime" , "Ice crystals forming a white deposit" .)