One who swings iron of a length to fit in working forge (6)
I believe the answer is:
golfer
'one who swings iron' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'a length to fit in working forge' is the wordplay.
'a length' becomes 'l' (abbreviation in geometry).
'to fit in' indicates putting letters inside.
'working' indicates an anagram.
'forge' with letters rearranged gives 'gofer'.
'l' put into 'gofer' is 'GOLFER'.
'of' is the link.
Can you help me to learn more?
(Other definitions for golfer that I've seen before include "McGinley or Harrington, say" , "Driver, chipper and putter" , "Player with clubs and tees" , "Swinger on course" , "Player on links" .)