Half-hearted fellas at work? Not so (5)
I believe the answer is:
false
'not so' is the definition.
(I've seen this before)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for false that I've seen before include "Sham" , "Misleading" , "Incorrect" , "Spurious, untrue" , "Forged" .)