Forger's requirement sees knave regularly in trouble (5)
I believe the answer is:
anvil
'forger's requirement' is the definition.
(a blacksmith uses an anvil to forge)
'knave regularly in trouble' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'in' means one lot of letters goes inside another.
'trouble' becomes 'ail' (synonyms).
The alternate letters of 'knave' are 'nv'.
'nv' placed into 'ail' is 'ANVIL'.
'sees' is the link.
(Other definitions for anvil that I've seen before include "Bone" , "Block on which metals are hammered" , "Smith's heavy iron block" , "Smithy's block" , "Block for metalworking" .)