One million quid put under lock and key (7)
I believe the answer is:
impound
'lock and key' is the definition.
I can't tell whether this defines the answer.
'one million quid put under' is the wordplay.
'one million' becomes 'im' (resembles 1m, 1 million).
'quid' becomes 'pound' (synonyms).
'put under' means one lot of letters go next to another.
'im'+'pound'='IMPOUND'
Can you help me to learn more?
(Other definitions for impound that I've seen before include "Shut up" , "Take property into legal custody" , "Sequester" , "Seize and take into legal custody" , "Take legal possession of" .)