Pair of warrant officers appearing in book (7)
I believe the answer is:
twosome
'pair' is the definition.
(I know that pair can be written as twosome)
'warrant officers appearing in book' is the wordplay.
'warrant officers' becomes 'WOs' (WO is short for warrant officer).
'appearing in' means one lot of letters goes inside another.
'book' becomes 'tome' (a tome is a book).
'wos' going within 'tome' is 'TWOSOME'.
'of' is the link.
(Other definitions for twosome that I've seen before include "Item" , "Pair, duo" , "match" , "game of golf" , "Couple" .)