King starts to underestimate ruling a large country (5)
I believe the answer is:
rural
'country' is the definition.
(similar in meaning)
'king starts to underestimate ruling a large' is the wordplay.
'king' becomes 'R' (abbreviation of Latin rex).
'starts to' indicates taking the first letters.
'large' becomes 'L' (eg in clothes sizes).
The initial letters of 'underestimate ruling' is 'ur'.
'r'+'ur'+'a'+'l'='RURAL'
(Other definitions for rural that I've seen before include "Rustic" , "Countrified" , "Cobbett saw on his rides?" , "Countrylike" , "Bucolic; pastoral" .)