Highly reactive element in the alkali metal group (7)
I believe the answer is:
caesium
'alkali metal group' is the definition.
I can't judge whether this defines the answer.
'highly reactive element in' is the wordplay.
'highly reactive' becomes 'a' (I can't explain this - if you can you should give a lot more credence to this answer).
'element' becomes 'cesium' (cesium is a kind of element).
'in' means one lot of letters goes inside another.
'a' going into 'cesium' is 'CAESIUM'.
'the' is the link.
Can you help me to learn more?
(Other definitions for caesium that I've seen before include "Rare metal element, Cs" , "Chemical element, symbol Cs" , "Metallic element with atomic number 55" , "Reactive metal element" .)