Soldiers escort old bishop in ceremonial dress (4)
I believe the answer is:
robe
'ceremonial dress' is the definition.
'soldiers escort old bishop' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'escort' indicates putting letters inside (one word carries or escorts another).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'bishop' becomes 'b' (abbreviation used in chess).
'o'+'b'='ob'
're' enclosing 'ob' is 'ROBE'.
'in' is the link.
(Other definitions for robe that I've seen before include "Loose garment for Boer" , "Formal attire" , "Loose outer garment" , "Gown, rich dress" , "Christening garment" .)