A number drink, we hear, and become strained (5,2)
I believe the answer is:
tense up
'become strained' is the definition.
'a number drink we hear and' is the wordplay.
I cannot quite see how this works, but
'number' could be 'e' (e is a kind of number) and 'e' is present in the answer.
'and' could be 'n' (common abbreviation for 'and') and 'n' is found within the answer.
'we hear' could be 'e' (I've seen this in other clues) and 'e' is found within the answer.
'drink' could be 'sup' (I've seen this before) and 'sup' is located in the remaining letters.
A single letter 't' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
(Other definitions for tense up that I've seen before include "become uneasy" , "Become stiff through stress or anxiety" , "become edgy" .)