Party head last seen in Truro (5)
I believe the answer is:
beano
'party' is the definition.
(I've seen this before)
'head last seen in truro' is the wordplay.
'head' becomes 'bean' (bean is a kind of head).
'last seen in' indicates one should take the final letters.
The final letter of 'truro' is 'o'.
'bean'+'o'='BEANO'
(Other definitions for beano that I've seen before include "Kids' comic" , "Celebration" , "Rowdy jollification" , "Informal party" , "A bone for a feast" .)