In Euston, especially, is it a gem? (5)
I believe the answer is:
stone
'a gem?' is the definition.
(I've seen this in another clue)
'in euston especially is it' is the wordplay.
'in' says the answer is hidden in the clue.
'STONE' is hidden within 'euston especially it'.
(Other definitions for stone that I've seen before include "Rock; fruit seed" , "Rock; cherry seed" , "Olive pip" , "Rag" , "14 lb" .)