Capital: something that constricts gains acquired (6)
I believe the answer is:
bogota
'capital something that' is the definition.
'Bogota' can be an answer for 'capital' (Bogota is an example). I'm not sure about the rest of the definition.
'constricts gains acquired' is the wordplay.
'constricts' becomes 'boa' (I am not sure about this - if you are sure you should believe this answer much more).
'gains' is an insertion indicator.
'acquired' becomes 'got' ('get' can be a synonym of 'acquire').
'boa' placed around 'got' is 'BOGOTA'.
Can you help me to learn more?
(Other definitions for bogota that I've seen before include "Columbian capital" , "Andean city" , "Colombian capital" , "Tobago in South America" , "Colombian city" .)