One and all — pick up rag if nervous (3,2,4)
I believe the answer is:
ill at ease
'nervous' is the definition.
(I've seen this before)
'one and all pick up rag' is the wordplay.
'one' becomes 'i' (roman numeral).
'and' says to put letters next to each other.
'pick up' shows that the letters should be reversed in order.
'rag' becomes 'tease' (teasing is a kind of ragging).
'all' back-to-front is 'lla'.
'i'+'lla'+'tease'='ILL AT EASE'
'if' is the link.
(Other definitions for ill at ease that I've seen before include "Uncomfortable with a situation" , "Fidgety" , "On edge" , "discomfited" , "Unable to relax" .)