Iron Man finally in Slough (3)
I believe the answer is:
fen
'slough' is the definition.
(I've seen this in another clue)
'iron man finally' is the wordplay.
'iron' becomes 'Fe' (chemical symbol for iron).
'man' becomes 'n' (knight, a man in chess).
'finally' says to put letters next to each other (some letters go at the end of others).
'fe'+'n'='FEN'
'in' acts as a link.
(Other definitions for fen that I've seen before include "Low marshy land" , "English marsh" , "Low-lying wet land with grassy vegetation" , "Low marshy area" , "Low-lying marshy area in England" .)