Sweetheart with strange dream about large gem (7)
I believe the answer is:
emerald
'gem' is the definition.
(emerald is a kind of gem)
'sweetheart with strange dream about large' is the wordplay.
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'with' is a charade indicator (letters next to each other).
'strange' indicates an anagram.
'about' indicates putting letters inside.
'large' becomes 'l'.
'dream' anagrammed gives 'merad'.
'merad' going around 'l' is 'merald'.
'e'+'merald'='EMERALD'
(Other definitions for emerald that I've seen before include "Precious green stone" , "Green variety of beryl" , "Green - jewel" , "Gem; type of moth" , "Green gemstone" .)