What could galvanise jazz in cafe bars? (4)
I believe the answer is:
zinc
'what could galvanise' is the definition.
(to galvanise is to coat in zinc)
'jazz in cafe bars?' is the wordplay.
'bars?' says the answer is hidden in the clue (the answer is imprisoned or barred in the clue).
'ZINC' is hidden within 'jazz in cafe'.
(Other definitions for zinc that I've seen before include "Blusish-white lustrous metallic element" , "It is used to coat iron against corrosion" , "Bluish-white metal" , "Antioxidant element" , "Metallic element present with copper in brass" .)