Mike, Trump's behind crowd (4)
I believe the answer is:
mass
'crowd' is the definition.
(I know that mass is a more specific form of the action crowd)
'mike trump's behind' is the wordplay.
'mike' becomes 'M' (phonetic alphabet: alpha, bravo, charlie etc.).
'trump's behind' becomes 'ass' (US term for your behind).
'm'+'ass'='MASS'
(Other definitions for mass that I've seen before include "Rite; quantity of matter" , "Christian ritual" , "Celebration of the Eucharist, especially in the RC Church" , "Roman Catholic service" , "Church service - collection" .)