Unpredictable boy holding sailor back (7)
I believe the answer is:
erratic
'unpredictable' is the definition.
(synonyms)
'boy holding sailor back' is the wordplay.
'boy' becomes 'eric' (I've seen this before).
'holding' means one lot of letters goes inside another.
'sailor' becomes 'tar' (slang term for a sailor).
'back' says the letters should be written backwards.
'tar' backwards is 'rat'.
'eric' placed around 'rat' is 'ERRATIC'.
(Other definitions for erratic that I've seen before include "Unpredictable, temperamental" , "Irregular in pattern or movement" , "Not regular in pattern" , "Mercurial" , "Deep affection" .)