Unreliable boy holding sailor up (7)
I believe the answer is:
erratic
'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 "Unpredictable; occasional" , "Fickle" , "Inconsistent, shifting" , "Irregular - eccentric" , "Shifting like Cartier" .)