Symbol of rank son has forged (4)
I believe the answer is:
sash
'symbol of rank' is the definition.
(part of a military uniform)
'son has forged' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'forged' indicates an anagram.
'has' with letters rearranged gives 'ash'.
's'+'ash'='SASH'
(Other definitions for sash that I've seen before include "Window frame; silk band" , "seen hanging around Miss World?" , "Decoration for the twelfth" , "Type of sliding window" , "Band - scarf" .)