Fold mat, put away alongside French horn (9)

'fold' is the definition.
(corrugating is a kind of folding)

'mat put away alongside french horn' is the wordplay.
'mat' becomes 'rug' (I've seen this before).
'put away' becomes 'ate' (put away can mean to eat).
'alongside' is a charade indicator (letters next to each other).
'french horn' becomes 'cor' ('horn' in French).
'rug'+'ate'='rugate'
'rugate' after 'cor' is 'CORRUGATE'.