Six-footers, like some Russian tsars (4)
I believe the answer is:
ants
'six-footers' is the definition.
(I've seen this in another clue)
'some russian tsars' is the wordplay.
'some' says the answer is hidden in the clue (some parts of the word are used).
'ANTS' can be found hidden inside 'russian tsars'.
'like' is the link.
(Other definitions for ants that I've seen before include "Emmets" , "They live in a formicary" , "Inhabitants of a formicary" , "Hard workers" , "Six-foot insects" .)