Singer — a lieutenant, old (4)
I believe the answer is:
alto
'singer' is the definition.
(alto is a kind of singer)
'a lieutenant old' is the wordplay.
'lieutenant' becomes 'lt'.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'a'+'lt'+'o'='ALTO'
(Other definitions for alto that I've seen before include "Counter-tenor or contralto" , "Singing range" , "Lowest female singing voice" , "Low female singing voice" , "Second of four parts (in singing)" .)