Swim underwater in small country (5)
I believe the answer is:
scuba
'swim underwater in' is the definition.
I know that 'underwater' relates to this answer. I also know that both the answer and definition are singular nouns
'small country' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'country' becomes 'cuba' (Cuba is an example).
's'+'cuba'='SCUBA'
Can you help me to learn more?
(Other definitions for scuba that I've seen before include "Apparatus for divers" , "Skin-diver's device" , "Diving apparatus" , "breather" , "Self-contained underwater breathing apparatus (acronym)" .)