Best have bananas, your head is unstable (3,5)
I believe the answer is:
top heavy
'unstable' is the definition.
(thesaurus)
'best have bananas your head' is the wordplay.
'best' becomes 'top' (top can mean the very best).
'bananas' is an anagram indicator (informal term for mad).
'head' suggests taking the first letters.
The first letter of 'your' is 'y'.
'have' with letters rearranged gives 'heav'.
'top'+'heav'+'y'='TOP-HEAVY'
'is' is the link.
(Other definitions for top heavy that I've seen before include "in danger of being brought down?" , "Likely to fall" , "administratively overburdened" .)