Type of immunity said to be picked up (4)
I believe the answer is:
herd
'type of immunity' is the definition.
(herd immunity)
'said to be picked up' is the wordplay.
'said to be' indicates a 'sounds like' (homophone) clue.
'picked up' becomes 'heard' (hearing is a kind of picking up).
'heard' sounds like 'HERD'.
(Other definitions for herd that I've seen before include "Stock of cattle" , "Large group of animals" , "Drive" , "Assemblage" , "cattle together" .)