Extensive pit retained by Wales somehow (9)
I believe the answer is:
wholesale
'extensive' is the definition.
(I've seen this before)
'pit retained by wales somehow' is the wordplay.
'pit' becomes 'hole' (a pit is a hole).
'retained by' means one lot of letters goes inside another.
'somehow' indicates an anagram.
'wales' is an anagram of 'wsale'.
'hole' put within 'wsale' is 'WHOLESALE'.
(Other definitions for wholesale that I've seen before include "sweeping" , "Business of supplying retailers" , "In large quantities - indiscriminately" , "In an indiscriminate way" , "extensively" .)