Carefully considering one’s options, unlike Charles the Fat, say? (8)
I believe the answer is:
thinking
'carefully considering one's options' is the definition.
(thinking about one's options)
'unlike charles the fat say?' is the wordplay.
I cannot really see how this works, but
'charles' could be 'king' (Charles I or Charles II) and 'king' is located in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found in the answer.
The remaining letters 'hin' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
(Other definitions for thinking that I've seen before include "consideration" , "Considering in the mind" , "Thought" , "Reasoning in the mind" , "Pondering" .)