Till the time when a letter comes from a London park (2,2)
I believe the answer is:
up to
'till' is the definition.
(thesaurus)
'time when a letter comes from a london park' is the wordplay.
I cannot quite see how this works, but
'time' could be 't' (abbreviation) and 't' is located in the answer.
'letter' could be 'u' and 'u' is found within the answer.
'from' could be 'o' (I've seen this in other clues) and 'o' is found within the answer.
'park' could be 'p' (abbreviation as on an automatic car's gear lever) and 'p' is located in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'the' acts as a link.
Can you help me to learn more?
(Other definitions for up to that I've seen before include "Engaged in doing" , "As far as a certain point" , "Busy with" , "As far as or as many as" , "As many as, though no more" .)