Having the top flat? That's sensible (5-6)
I believe the answer is:
level-headed
'sensible' is the definition.
(I've seen this before)
'having the top flat?' is the wordplay.
I cannot quite see how this works, but
'top' could be 'head' (head is a kind of top) and 'head' is found in the answer.
'flat?' could be 'level' ('level' can be a synonym of 'flat') and 'level' is found in the answer.
The remaining letters 'ed' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for level-headed that I've seen before include "Showing good judgement" , "Calm and sensible" .)