Man, say, behind source of ludicrous yarn (5)
I believe the answer is:
lisle
'yarn' is the definition.
(I know that lisle is a type of yarn)
'man say behind source of ludicrous' is the wordplay.
'man say' becomes 'isle' (Isle of Man is an example).
'behind' says to put letters next to each other.
'source of' indicates taking the first letters (the start - or source - of the word).
The initial letter of 'ludicrous' is 'l'.
'isle' after 'l' is 'LISLE'.
(Other definitions for lisle that I've seen before include "Maker of hose" , "Strong cotton fabric" , "Strong tightly twisted cotton thread" , "Strong twisted cotton thread for the strange Ellis" , "Twisted yarn" .)