Beg to have one extra place reserved (7)
I believe the answer is:
implore
'beg' is the definition.
(I know that beg can be written as implore)
'one extra place reserved' is the wordplay.
I cannot quite see how this works, but
'one' could be 'o' (I've seen this in other clues) and 'o' is found in the answer.
'place' could be 'pl' (abbreviation used in postal addresses) and 'pl' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'to have' is the link.
(Other definitions for implore that I've seen before include "Ask earnestly" , "Beg desperately" , "press" , "Beg earnestly, beseech" , "More lip (anag)" .)