Thought one needs a spare sovereign (8)
I believe the answer is:
thinking
'thought one' is the definition.
'thinking' can be an answer for 'thought' (I've seen this before). I am not sure about the 'one' bit.
'a spare sovereign' is the wordplay.
'a spare' becomes 'thin' (I've seen this in another clue).
'sovereign' becomes 'king' (king is a kind of sovereign).
'thin'+'king'='THINKING'
'needs' is the link.
Can you help me to learn more?
(Other definitions for thinking that I've seen before include "Considering in the mind" , "Cogitating" , "Take" , "Mental process" , "Thought" .)