Place near London, order fish there mostly for distribution (13)
I believe the answer is:
hertfordshire
'place near london' is the definition.
(county near London)
'order fish there mostly for distribution' is the wordplay.
'mostly' means to remove the last letter.
'for distribution' indicates anagramming the letters (I've seen this in other clues).
'order'+'fish'+'there'='orderfishthere'
'orderfishthere' with its last letter removed is 'orderfishther'.
'orderfishther' is an anagram of 'HERTFORDSHIRE'.
(Other definitions for hertfordshire that I've seen before include "A home county" , "County of Watford and Ware" , "County to N. of London" , "Watford's county" , "Southern county" .)