Deceive the first man (5)
I believe the answer is:
trick
'deceive' is the definition.
(I know that trick is a more specific form of deceive)
'the first man' is the wordplay.
'the first' becomes 't' (rst letter of 'the').
'man' becomes 'rick' ().
't'+'rick'='TRICK'
(Other definitions for trick that I've seen before include "Strategem" , "stunt" , "Subterfuge" , "Point gained in bridge - a deception?" , "Magician's ploy" .)