Achievement of force invading eastern stronghold (6)
I believe the answer is:
effort
'achievement' is the definition.
(effort is a kind of achievement)
'force invading eastern stronghold' is the wordplay.
'force' becomes 'F' (symbol used in Physics e.g. f=ma).
'invading' indicates putting letters inside.
'eastern' becomes 'e'.
'stronghold' becomes 'fort' (I've seen this before).
'e'+'fort'='efort'
'f' inserted inside 'efort' is 'EFFORT'.
'of' acts as a link.
(Other definitions for effort that I've seen before include "Work - application" , "Exertion, elbow grease" , "Vigorous attempt" , "Exertion - attempt - achievement" , "Try - labour - pains" .)