We'll do so as a half promise (3)
I believe the answer is:
see
'we'll do' is the definition.
I can't tell whether this defines the answer.
'as a half promise' is the wordplay.
'a half' means to take half the letters.
'promise' becomes 'ee' (I can't explain this - if you can you should give a lot more credence to this answer).
'as' halved is 's'.
's'+'ee'='SEE'
'so' acts as a link.
Can you help me to learn more?
(Other definitions for see that I've seen before include "Appreciate" , "Bishop's diocese" , "twig" , "make out" , "ecclesiastical division" .)