The principal can make a good man turn pale (6)
I believe the answer is:
staple
'the principal' is the definition.
(I've seen this before)
'make a good man turn pale' is the wordplay.
'make' is an anagram indicator.
'a good man' becomes 's'.
'turn' becomes 't' (abbreviation in Oxford Dictionary of Abbreviations).
's'+'t'+'pale'='stpale'
'stpale' is an anagram of 'STAPLE'.
'can' acts as a link.
(Other definitions for staple that I've seen before include "Principle - metal fastener" , "Clip" , "Basic element" , "Main - way to fasten paper" , "e.g. rice, sweetcorn ..." .)