During climb, I begin to drink (6)
I believe the answer is:
imbibe
'to drink' is the definition.
(synonyms)
'during climb i begin' is the wordplay.
'during' says the answer is hidden in the clue.
'IMBIBE' is hidden in the letters of 'climb i begin'.
(Other definitions for imbibe that I've seen before include "Swallow" , "Quaff" , "Tope" , "Partake of drink" , "Drink, take in" .)