Policy group’s tip for fitting out aquarium? (5,4)
I believe the answer is:
think tank
'policy group's tip' is the definition.
I can't judge whether this defines the answer.
'fitting out aquarium?' is the wordplay.
'fitting out' becomes 'think' (I can't explain this - if you can you should believe this answer much more).
'aquarium?' becomes 'tank' (aquarium is a kind of tank).
'think'+'tank'='THINK-TANK'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for think tank that I've seen before include "source of ideas" , "Group of experts considering specific problems" , "Brains trust" , "It researches and reports on policy issues" , "Group of specialists considering problem(s)" .)