Barrier to picking up what's in Panama disease (3)
I believe the answer is:
dam
'barrier' is the definition.
(dam is a kind of barrier)
'picking up what's in panama disease' is the wordplay.
'picking up' says the letters should be written backwards.
'what's in' indicates a hidden word.
'mad' is hidden in the letters of 'panama disease'.
'mad' reversed gives 'DAM'.
'to' is the link.
(Other definitions for dam that I've seen before include "Female parent of an animal, especially livestock" , "A mad kind of mum" , "Mother - barrier" , "Embankment" , "Female parent; obstruction" .)