Principal thug somehow unstable (3-5)
I believe the answer is:
top-heavy
'somehow unstable' is the definition.
'top heavy' can be an answer for 'unstable' (thesaurus). I am unsure of the 'somehow' bit.
'principal thug' is the wordplay.
'principal' becomes 'top' (I've seen this in another clue).
'thug' becomes 'heavy' (both can mean a violent or aggressive person).
'top'+'heavy'='TOP-HEAVY'
Can you help me to learn more?
(Other definitions for top-heavy that I've seen before include "Unstable through being over-loaded at the higher end" , "Likely to fall over" , "Unstable through an excess of weight" , "Unstable through weight distribution" , "(Of company) having too many executives" .)