One of the main forces in the world (3,5)
I believe the answer is:
sea power
'one of' is the definition.
The definition suggests a singular noun which matches the answer.
'main forces in the world' is the wordplay.
'main' becomes 'sea' (the main is an old name for the sea).
'forces in the world' becomes 'power' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'sea'+'power'='SEA-POWER'
'the' acts as a link.
Can you help me to learn more?
(Other definitions for sea power that I've seen before include "Naval strength" , "threat, in the main" , "Ability to rule the waves?" .)