Expert's love for top American mailman? (10)
I believe the answer is:
postmaster
'american mailman?' is the definition.
(position in American postal services)
'expert's love for top' is the wordplay.
I cannot quite see how this works, but
'love' could be 'o' (love means zero in tennis) and 'o' is present in the answer.
'expert' could be 'master' (master is a kind of expert) and 'master' is present in the answer.
The remaining letters 'pst' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
(Other definitions for postmaster that I've seen before include "who runs a local office?" , "Mail chief" .)