Lately Ukrainians stop aggressive force westward showing rapidity (5)
I believe the answer is:
speed
'rapidity' is the definition.
(I've seen this in another clue)
'lately ukrainians stop aggressive force westward' is the wordplay.
'lately' says to take the final letters (only the latest - last - letters are used).
The final letters of 'ukrainians stop aggressive force westward' are 'SPEED'.
'showing' is the link.
(Other definitions for speed that I've seen before include "Swiftness" , "Expedition" , "The going rate" , "Tempo - drug" , "Rapidity - amphetamine" .)