Express: 'Putin's beginning to be involved in invasion' (5)
I believe the answer is:
rapid
'express' is the definition.
(thesaurus)
'putin's beginning to be involved in invasion' is the wordplay.
'beginning to' indicates taking the first letters.
'be involved in' is an insertion indicator.
'invasion' becomes 'raid' (I've seen this before).
The initial letter of 'putin' is 'p'.
'p' placed inside 'raid' is 'RAPID'.
(Other definitions for rapid that I've seen before include "moving fast" , "Prompt" , "Speedy, quick" , "Expeditious" , "Precipitate" .)