Really try in a TT (2,5)
I believe the answer is:
at heart
'really' is the definition.
(I've seen this in another clue)
'try in a tt' is the wordplay.
'try' becomes 'hear' (as in to try someone in court or hear their case).
'in' indicates putting letters inside.
'a'+'tt'='att'
'hear' put into 'att' is 'AT HEART'.
(Other definitions for at heart that I've seen before include "Fundamentally" , "Really" , "In reality" , "Essentially" , "In essence" .)