Think tank ultimately concerned with Conservative squeezing (6)
I believe the answer is:
reckon
'think' is the definition.
(I know that think can be written as reckon)
'tank ultimately concerned with conservative squeezing' is the wordplay.
'ultimately' says to take the final letters.
'concerned with' becomes 're' (re can mean regarding, concerning).
'conservative' becomes 'con' (abbreviation for Conservative).
'squeezing' is an insertion indicator (squeeze can mean hug or embrace).
The final letter of 'tank' is 'k'.
're'+'con'='recon'
'k' inserted inside 'recon' is 'RECKON'.
(Other definitions for reckon that I've seen before include "Calculate or opine" , "Consider, judge to be probable" , "Calculate, compute or estimate" , "Think - calculate" , "Estimate - judge" .)