With pleasure, serving American mushrooms, for example (5)
I believe the answer is:
fungi
'mushrooms for example' is the definition.
(mushroom is a kind of fungus)
'with pleasure serving american' is the wordplay.
'with' is a charade indicator (letters next to each other).
'pleasure' becomes 'fun' (I've seen this before).
'serving american' becomes 'gi' (I've seen this in other clues).
'fun'+'gi' is 'FUNGI'.
(Other definitions for fungi that I've seen before include "Eg, toadstools" , "Spore-producing organisms" , "Toadstools, say" , "Magic mushrooms perhaps" , "Yeast, etc" .)