Foolish king within almost too attentive (3,4)
I believe the answer is:
all ears
'attentive' is the definition.
(I've seen this in another clue)
'foolish king within almost too' is the wordplay.
'foolish king' becomes 'Lear' (reference to King Lear).
'within' is an insertion indicator.
'almost' means to remove the last letter (most of the word but not all of it).
'too' becomes 'also' (synonyms).
'also' with its final letter removed is 'als'.
'lear' going inside 'als' is 'ALL EARS'.
(Other definitions for all ears that I've seen before include "Listening attentively" , "description of cornfield?" , "Very alert" , "Prepared to listen" , "Being very attentive" .)