Note turned up concealed by welcome mat (4)
I believe the answer is:
memo
'note' is the definition.
(memo is a kind of note)
'turned up concealed by welcome mat' is the wordplay.
'turned up' is a reversal indicator.
'concealed' indicates the answer is hidden within the clue.
'omem' can be found hidden inside 'by welcome mat'.
'omem' written backwards gives 'MEMO'.
(Other definitions for memo that I've seen before include "diplomatic communication?" , "A little something to be remembered" , "Businesslike message" , "In brief, a note of events" , "Written business message, in short" .)