Number forming a herd around one French trading centre (7)
I believe the answer is:
hundred
'number' is the definition.
(I know that hundred is a type of large integer)
'forming a herd around one french trading centre' is the wordplay.
'forming' indicates an anagram.
'around' means one lot of letters goes inside another.
'one french' becomes 'un' ('one' in French).
'centre' means to look at the middle letters.
The centre of 'trading' is 'd'.
'herd' with letters rearranged gives 'hdre'.
'hdre' enclosing 'un' is 'hundre'.
'hundre'+'d'='HUNDRED'
(Other definitions for hundred that I've seen before include "Ancient division of a county" , "Number; county subdivision" , "Square number" , "Former division of county" , "Historical division of a county - number" .)