A little drink that is - yes, soft (3)
I believe the answer is:
sip
'a little drink' is the definition.
(I've seen this in another clue)
'yes soft' is the wordplay.
'yes' becomes 'si' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'soft' becomes 'p' (musical abbreviation for piano, 'soft').
'si'+'p'='SIP'
'that is' acts as a link.
Can you help me to learn more?
(Other definitions for sip that I've seen before include "Smallest drink" , "Drink slowly" , "A small taste" , "Tiny amount of drink" , "Take small mouthfuls of (drink)" .)