Stone in ring father left (4)
I believe the answer is:
opal
'stone' is the definition.
(gemstone)
'ring father left' is the wordplay.
'ring' becomes 'O' (letter resembling a ring).
'father' becomes 'pa' (pa can mean dad or father).
'left' becomes 'l' (common abbreviation).
'o'+'pa'+'l'='OPAL'
'in' acts as a link.
(Other definitions for opal that I've seen before include "what may be set in 29 [DIAL]" , "One in a ring perhaps" , "Milky white gem" , "Jewellery item" , "Variegated gemstone" .)