More discerning, irrational and cold, Starmer turns guts (7)

'more discerning' is the definition.
Both the answer and definition are comparative adjectives.
Maybe there's a link between them I don't understand?

'irrational and cold starmer turns guts' is the wordplay.
I cannot quite see how this works, but
'irrational' could be 'pi' (an irrational number) and 'pi' is located in the answer.
'cold' could be 'c' (eg on taps) and 'c' is located in the answer.
The remaining letters 'kier' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?