Slip in modest drink (6)
I believe the answer is:
sherry
'drink' is the definition.
(sherry is a kind of drink)
'slip in modest' is the wordplay.
'slip' becomes 'err' (to err is to make an error).
'in' indicates putting letters inside.
'modest' becomes 'shy'.
'err' going within 'shy' is 'SHERRY'.
(Other definitions for sherry that I've seen before include "Fortified wine originally from Spain" , "Spanish fortified wine" , "Wine from the Jerez region of Spain" , "Christmas drink?" , "Oloroso or amontillado?" .)