Realise the weekend is today! (4)
I believe the answer is:
know
'realise' is the definition.
(I know that know is a more specific form of the action realise)
'weekend is today' is the wordplay.
'weekend' becomes 'k' (I've seen this in other clues).
'is today' becomes 'now' (I've seen this before).
'k'+'now'='KNOW'
'the' is the link.
(Other definitions for know that I've seen before include "Be familiar with" , "Recognize" , "Recognise" , "Have information" , "Be sure of something" .)