Sovereign is underweight considering (8)
I believe the answer is:
thinking
'considering' is the definition.
The answer and definition can be both related to thinking as well as being present participle verbs.
Perhaps you can see a link between them that I can't see?
'sovereign is underweight' is the wordplay.
I cannot really see how this works, but
'sovereign' could be 'king' (king is a kind of sovereign) and 'king' is present in the answer.
'underweight' could be 'thin' (similar in meaning) and 'thin' is found within the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for thinking that I've seen before include "Considering in the mind" , "Take" , "rationale" , "Mental process" , "Cogitating" .)