As you are soundly taking last place after me, estimate how long (7)
I believe the answer is:
'last place after me estimate how long' is the definition.
I can't tell whether this defines the answer.
'as you are soundly taking' is the wordplay.
'you' becomes 'u' (commonly used in online communication).
'are soundly' becomes 'mere' (I am not sure about this - if you are sure you should believe this answer much more).
'taking' is an insertion indicator.
'asu' going within 'mere' is 'MEASURE'.
Can you help me to learn more?
(Other definitions for measure that I've seen before include "Assess length or height" , "Gauge" , "Unit of verse" , "Get hands, perhaps" , "Standard unit; dance" .)