Unreliable boy holding sailor up (7)
I believe the answer is:
Here is my best explanation:
'unreliable' is the definition.
('erratic' can be similar in meaning to 'unreliable')
'boy holding sailor up' is the wordplay.
'boy' becomes 'eric' (I've seen this before**).
'holding' means one lot of letters goes inside another.
'sailor' becomes 'tar' (tar is a kind of sailor).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'tar' reversed gives 'rat'.
'eric' going around 'rat' is 'ERRATIC'.
(Other definitions for erratic that I've seen before include "Shifting like Cartier", "Variable", "Inconsistent, shifting", "Inconstant", "Fickle", "Deep affection", "Liable to sudden unpredictable change", "Irregular and unreliable", "Not even" and "Shifting, unpredictable".)