Fathead accustomed to being overloaded! (5)
I believe the answer is:
fused
'being overloaded' is the definition.
Both the definition and answer are adjectives. Maybe there's an association between them I don't understand?
'fathead accustomed' is the wordplay.
'fathead' becomes 'f' (head letter of 'fat').
'accustomed' becomes 'used' (I've seen this before).
'f'+'used'='FUSED'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for fused that I've seen before include "Melted together" , "Joined by melting" , "Combined" , "Welded together by heat" , "Stuck together" .)