Testify with very pained expression? (5)
I believe the answer is:
vouch
'testify' is the definition.
(I know that vouch is a more specific form of testify)
'with very pained expression?' is the wordplay.
'with' means one lot of letters go next to another.
'very' becomes 'v' (common abbreviation).
'pained expression?' becomes 'ouch' (I've seen this in other clues).
'v'+'ouch' is 'VOUCH'.
(Other definitions for vouch that I've seen before include "Be surety (for)" , "warrant" , "Declare support" , "Testify" , "Give personal assurance; guarantee" .)