One piercing awfully long tongue (5)
I believe the answer is:
lingo
'tongue' is the definition.
(I've seen this before)
'one piercing awfully long' is the wordplay.
'one' becomes 'I' (Roman numeral).
'piercing' indicates putting letters inside (some letters pierce a hole in others).
'awfully' is an anagram indicator.
'long' is an anagram of 'lngo'.
'i' inserted within 'lngo' is 'LINGO'.
(Other definitions for lingo that I've seen before include "Johnny Foreigner speaks one" , "Specialised vocabulary" , "Vernacular" , "Language (slang)" , "speech" .)