Research group’s rare function embraced by thousands (5-4)
I believe the answer is:
think-tank
'research group's' is the definition.
(group carrying out research)
'rare function embraced by thousands' is the wordplay.
'rare' becomes 'thin' (similar in meaning).
'function' becomes 'tan' (tangent, mathematical function).
'embraced by' is an insertion indicator.
'thousands' becomes 'KK' (K can mean thousand, e.g. t0k).
'tan' going into 'kk' is 'ktank'.
'thin'+'ktank'='THINK-TANK'
(Other definitions for think-tank that I've seen before include "Group of experts considering specific problems" , "Group set up to consider various problems" , "Group of specialists considering problem(s)" , "Group of experts providing advice, often on national matters" , "It researches and reports on policy issues" .)