Royal has given up lying - finally? It's true (4)
I believe the answer is:
real
'true' is the definition.
(similar in meaning)
'royal has given up lying finally?' is the wordplay.
'royal' becomes 'r' (common abbreviation e.g. RM - Royal Marine).
'has' means one lot of letters go next to another.
'given up' shows that the letters should be reversed in order (in a down clue, letters go up).
'lying finally?' becomes 'lae' (I can't explain this - if you can you should give a lot more credence to this answer).
'lae' in reverse letter order is 'eal'.
'r'+'eal'='REAL'
'it's' is the link.
Can you help me to learn more?
(Other definitions for real that I've seen before include "very American" , "Authentic" , "True" , "Actual and tangible" , "Physically existing" .)