Support or prohibit defending base (4)
I believe the answer is:
bear
'support' is the definition.
(bear can mean to support or carry)
'prohibit defending base' is the wordplay.
'prohibit' becomes 'bar' (to bar is to ban or prohibit).
'defending' means one lot of letters goes inside another (inserted letters are defended).
'base' becomes 'e' (Euler's number - base of natural logarithms).
'bar' going around 'e' is 'BEAR'.
'or' is the link.
(Other definitions for bear that I've seen before include "Pessimistic investor" , "Carry - teddy" , "Tolerate - carnivore" , "Exhibit" , "Carry or endure" .)