Do evening work that could be heartily boring! (4)
I believe the answer is:
iron
'do evening work' is the definition.
(to iron is to even out clothing)
'could be heartily boring' is the wordplay.
'could be' is an anagram indicator.
'heartily' means to look at the middle letters.
The middle letters of 'boring' are 'orin'.
'orin' is an anagram of 'IRON'.
'that' is the link.
(Other definitions for iron that I've seen before include "Metallic element, symbol Fe" , "stern" , "Main element in steel" , "1 down, symbol Fe" , "Smooth - metal" .)