Some unhealthy phenomenon that’s in centre of Guinea-Bissau (6)
I believe the answer is:
hyphen
'that's in centre of guinea-bissau' is the definition.
(Guinea-Bissau has a hyphen in the middle)
'some unhealthy phenomenon' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'HYPHEN' can be found hidden inside 'unhealthy phenomenon'.
(Other definitions for hyphen that I've seen before include "A horizontal words-joiner" , "Line in a double-barrelled name, say" , "Mark joining two words" , "Word-joining mark" , "seen in T-shirt and G-string" .)