Remember to phone back (6)
I believe the answer is:
recall
'remember' is the definition.
(to recall something is to remember it)
'phone back' is the wordplay.
I cannot quite understand how this works, but
'phone' could be 'call' (synonyms) and 'call' is located in the answer.
The remaining letters 're' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'to' acts as a link.
Can you help me to learn more?
(Other definitions for recall that I've seen before include "Recollect" , "Summon back as to memory" , "Get back" , "Don''t forget - come again" , "Summon back or bring to mind" .)