A forced entry may be in vain, so planned (8)
I believe the answer is:
invasion
'a forced entry' is the definition.
'in vain so planned' is the wordplay.
'planned' indicates an anagram.
'vain'+'so'='vainso'
'vainso' is an anagram of 'vasion'.
'in'+'vasion'='INVASION'
'may be' is the link.
(Other definitions for invasion that I've seen before include "Encroachment by an army" , "Incursive attack" , "Unwelcome intrusion" , "Entry by force by an army" , "Occupation" .)