Send friendly message with well-penned substance, perhaps (4)
I believe the answer is:
wink
'send friendly message' is the definition.
'with well-penned substance perhaps' is the wordplay.
'with' becomes 'w' (abbreviation).
'well-penned substance perhaps' becomes 'ink' (substance often used with pens).
'w'+'ink'='WINK'
(Other definitions for wink that I've seen before include "in a trice" , "Sleep moment; twinkle" , "Lower eyelid briefly" , "... associated eye movement" , "It may be tipped" .)