To be used in a fatheaded piece of work (3)
I believe the answer is:
for
'to' is the definition.
(thesaurus)
'piece of work' is the wordplay.
'piece' becomes 'r' (abbreviation for Rook).
'work' indicates an anagram.
'r'+'of'='rof'
'rof' with letters rearranged gives 'FOR'.
'be used in a fatheaded' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for for that I've seen before include "On behalf of" , "and 16 and 20: Notwithstanding" , "See 24" , "In support of" , "Favouring" .)