Cross, if taken inside to give evidence (7)
I believe the answer is:
testify
'give evidence' is the definition.
(I've seen this before)
'cross if taken inside' is the wordplay.
'cross' becomes 'testy' (I've seen this in another clue).
'taken inside' means one lot of letters goes inside another.
'testy' going around 'if' is 'TESTIFY'.
'to' is the link.
(Other definitions for testify that I've seen before include "Give evidence in court" , "Bear witness in court" , "Affirm (on oath)" , "sworn" , "take the stand" .)