Truth from Conservative in blubber (4)
I believe the answer is:
fact
'truth' is the definition.
(I've seen this before)
'conservative in blubber' is the wordplay.
'conservative' becomes 'C' (abbreviation commonly used in UK politics).
'in' means one lot of letters goes inside another.
'blubber' becomes 'fat' (similar in meaning).
'c' placed inside 'fat' is 'FACT'.
'from' is the link.
(Other definitions for fact that I've seen before include "Truth, reality" , "Undisputed truth" , "Fait accompli" , "Not fake news!" , "matter of record" .)