Second magnitude star is swelling (7)
I believe the answer is:
blister
'is swelling' is the definition.
(thesaurus)
'second magnitude star' is the wordplay.
I cannot quite understand how this works, but
'second' could be 's' (common abbreviation) and 's' is present in the answer.
'magnitude' could be 'r' (r is a kind of magnitude) and 'r' is present in the answer.
The remaining letters 'blite' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for blister that I've seen before include "Sore bubble on skin" , "Result of friction" , "Friction bubble on skin" , "Thin bubble on the skin" , "Bubble raised on the skin" .)