Give evidence providing observer in hearing supports trial (7)
I believe the answer is:
testify
'give evidence' is the definition.
(I've seen this before)
'providing observer in hearing supports trial' is the wordplay.
'providing' becomes 'if' (eg 'providing it's sunny' means 'if it's sunny').
'observer in hearing' becomes 'y' (I can't justify this - if you can you should give a lot more credence to this answer).
'supports' means one lot of letters go next to another (bottom letters provide a support for the top ones).
'trial' becomes 'test' (synonyms).
'if'+'y'='ify'
'ify' after 'test' is 'TESTIFY'.
Can you help me to learn more?
(Other definitions for testify that I've seen before include "Bear witness in court" , "Affirm (on oath)" , "take the stand" , "Give evidence in court" , "sworn" .)