Let the cat out of the bag at the British laboratory (4)
I believe the answer is:
blab
'let the cat out of the bag' is the definition.
(I've seen this in another clue)
'at the british laboratory' is the wordplay.
'at' says to put letters next to each other.
'the british' becomes 'b' (abbreviation e.g. in 'BBC').
'laboratory' becomes 'lab' (synonyms).
'b' put next to 'lab' is 'BLAB'.
(Other definitions for blab that I've seen before include "Let slip" , "Talk indiscreetly" , "Tells tales" , "Tell tales" , "Divulge secret information" .)