Reg only has selfish consideration (3)
I believe the answer is:
ego
'selfish consideration' is the definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Maybe you can see a link between them that I don't see?
'reg only has' is the wordplay.
'has' says the answer is hidden in the clue.
'EGO' can be found hidden inside 'reg only'.
Can you help me to learn more?
(Other definitions for ego that I've seen before include "Sense of self, conceit" , "One's image of oneself" , "Consciousness" , "(Self-assertive) personality" , "number one" .)