First, play the part of Simon Templar? (4)
I believe the answer is:
best
'first' is the definition.
(similar in meaning)
'play the part of simon templar?' is the wordplay.
'play' becomes 'be' (as in playing a character).
'the part of' suggests taking the first letters (I've seen 'part' mean this).
The first letters of 'simon templar' is 'st'.
'be'+'st'='BEST'
(Other definitions for best that I've seen before include "George -, footballer" , "BOSOM] ?" , "Most suitably" , "most fitting" , "Most outstanding" .)