Unreliable half-back in the line (6)
I believe the answer is:
fickle
'unreliable' is the definition.
(I've seen this before)
'half-back in the line' is the wordplay.
'half' suggests halving the letters.
'in the' means one lot of letters goes inside another.
'line' becomes 'file' (file is a kind of line).
'back' with half the letters taken is 'ck'.
'ck' going inside 'file' is 'FICKLE'.
(Other definitions for fickle that I've seen before include "Often changing, not constant or loyal" , "Often changing, not reliable emotionally" , "Subject to changes of mind" , "Inconstant, changeable" , "Liable to sudden unpredictable change" .)