Shaken by a crash outside the Turk’s Head, you need a stiffener (6)
I believe the answer is:
starch
'a stiffener' is the definition.
(I've seen this before)
'shaken by a crash outside the turk's head' is the wordplay.
'shaken' indicates an anagram.
'by' is a charade indicator (letters next to each other).
'outside' suggests removing the centre.
'head' indicates taking the first letters.
'crash' with its centre taken out is 'crsh'.
The first letter of 'turks' is 't'.
'a'+'crsh'='acrsh'
'acrsh' after 't' is 'tacrsh'.
'tacrsh' is an anagram of 'STARCH'.
'you need' is the link.
(Other definitions for starch that I've seen before include "Stiffness of manner" , "Stuff that stiffens, say collars" , "Carbohydrate; stiffen" , "Substance found mainly in potatoes and cereals" , "Stiffness; carbohydrate" .)