Jack’s entering bar with old sort of guitar (5)
I believe the answer is:
banjo
'sort of guitar' is the definition.
'jack's entering bar with old' is the wordplay.
'jack' becomes 'j' (playing card abbreviation).
'entering' means one lot of letters goes inside another.
'bar' becomes 'ban' (I've seen this in other clues).
'with' is a charade indicator (letters next to each other).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'ban'+'o'='bano'
'j' placed within 'bano' is 'BANJO'.
(Other definitions for banjo that I've seen before include "Stringed instrument with long neck" , "Formby's instrument" , "Relative of the guitar" , "Small plucked instrument" , "Instrument with long neck and round body" .)