Petitions soldier perhaps to be nice (8)
I believe the answer is:
pleasant
'nice' is the definition.
(similar in meaning)
'petitions soldier perhaps' is the wordplay.
'petitions' becomes 'pleas' (I've seen this in another clue).
'soldier perhaps' becomes 'ant' (soldier is a type of ant).
'pleas'+'ant'='PLEASANT'
'to be' acts as a link.
(Other definitions for pleasant that I've seen before include "A greeable, affable" , "Likeable, friendly" , "Nice, satisfying" , "Lovely" , "Delightful" .)