Old German boy has stolen a kiss (5)
I believe the answer is:
saxon
'old german' is the definition.
(I've seen this before)
'boy has stolen a kiss' is the wordplay.
'boy' becomes 'son' (synonyms).
'has stolen' means one lot of letters goes inside another (inserted letters are stolen).
'kiss' becomes 'x' (X can represent a kiss).
'a'+'x'='ax'
'son' going around 'ax' is 'SAXON'.
(Other definitions for saxon that I've seen before include "Member of Germanic tribe settled in southern England after the Romans" , "Settler from Germany in the home counties" , "Early invader of southern England" , "Race of England's rulers before the Conquest" , "German invader of Britain" .)