Jefferson, Nixon, Truman: to their friends perhaps ordinary people (3,4,3,5)
I believe the answer is:
tom dick and harry
'ordinary people' is the definition.
(I've seen this before)
'jefferson nixon truman to their' is the wordplay.
'jefferson' becomes 'Tom' (as in Thomas Jefferson).
'nixon' becomes 'Dick' (as in Richard Nixon - 'Tricky Dick').
'truman' becomes 'Harry' (US president Harry S Truman).
'to' says to put letters next to each other (I've seen this in other clues).
'their' becomes 'and' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'harry' after 'and' is 'andharry'.
'tom'+'dick'+'andharry'='TOM DICK AND HARRY'
'friends perhaps' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for tom dick and harry that I've seen before include "Ordinary people" , "Anyone" , "Being basic geezer" .)