I call for help — Yankee sat back in balanced state (8)
I believe the answer is:
isostasy
'balanced state' is the definition.
(state of gravitational balance or equilibrium)
'i call for help yankee sat back in' is the wordplay.
'call for help' becomes 'sos' (I've seen this in another clue).
'yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
'back' is a reversal indicator.
'in' indicates putting letters inside.
'sat' back-to-front is 'tas'.
'sos'+'y'='sosy'
'sosy' going around 'tas' is 'sostasy'.
'i'+'sostasy'='ISOSTASY'
(Other definitions for isostasy that I've seen before include "Geological equilibrium" , "earthly balance" .)