It blasts them asunder but not same in return (7)
I believe the answer is:
thunder
'it blasts' is the definition.
The definition suggests a singular noun which matches the answer.
'them asunder but not same in return' is the wordplay.
'but not' is a deletion indicator.
'in return' says the letters should be written backwards (return can mean to go backwards).
'them'+'asunder'='themasunder'
'same' back-to-front is 'emas'.
'themasunder' with 'emas' taken out is 'THUNDER'.
Can you help me to learn more?
(Other definitions for thunder that I've seen before include "Reverberate" , "Weather featuring with 1" , "Noise named after the god Thor" , "Big noise" , "Loud rumbling noise" .)