Henry, aghast, changing label on social media (7)
I believe the answer is:
hashtag
'label on social media' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.
'henry aghast changing' is the wordplay.
'henry' becomes 'H' (symbol for the unit of electrical inductance).
'changing' is an anagram indicator.
'aghast' with letters rearranged gives 'ashtag'.
'h'+'ashtag'='HASHTAG'
Can you help me to learn more?
(Other definitions for hashtag that I've seen before include "Word or phrase to aid online searches - hag's hat (anag)" , "Symbol used to indicate the subject of a Twitter message" , "Twitter mark" , "Label on a message on Twitter" , "Octothorp" .)