Revolutionary Soviet leader beheaded in square (4)
I believe the answer is:
nine
'square' is the definition.
(nine is a square number)
'revolutionary soviet leader beheaded' is the wordplay.
'revolutionary' is a reversal indicator (letters revolve around).
'soviet leader' becomes 'lenin' (I've seen this before).
'beheaded' means to remove the first letter (the head of the word is removed).
'lenin' with its initial letter removed is 'enin'.
'enin' in reverse letter order is 'NINE'.
'in' is the link.
(Other definitions for nine that I've seen before include "Number of players in a baseball fielding team" , "A stitch in time saves them" , "Days Lady Jane 5 ac reigned" , "SECOND PART OF 14" , "Figure" .)