Amusing chap called for mushrooms (5)
I believe the answer is:
fungi
'mushrooms' is the definition.
(I've seen this before)
'amusing chap' is the wordplay.
'amusing' becomes 'fun' (synonyms).
'chap' becomes 'gi' (I can't explain this - if you can you should believe this answer much more).
'fun'+'gi'='FUNGI'
'called for' acts as a link.
Can you help me to learn more?
(Other definitions for fungi that I've seen before include "Yeast, etc" , "Mushrooms, toadstools, moulds etc" , "Yeasts, moulds, mushrooms etc" , "Kingdom of lower plants" , "Spore producers" .)