Number nine, peg between two poles (7)
I believe the answer is:
sixteen
'number' is the definition.
(sixteen is a kind of number)
'nine peg between two poles' is the wordplay.
'nine' becomes 'IX' (roman numeral).
'peg' becomes 'tee' (tee is a kind of peg).
'between' means one lot of letters goes inside another.
'two poles' becomes 'sn'.
'ix'+'tee'='ixtee'
'ixtee' inserted within 'sn' is 'SIXTEEN'.
(Other definitions for sixteen that I've seen before include "4 x 4" , "Sweet Little -, (Chuck Berry)" , ""Sweet" age?" , "XVI" , "8 x 2" .)