Again one had left alone, it's true (4)
I believe the answer is:
real
'true' is the definition.
(similar in meaning)
'again one had left' is the wordplay.
'again' becomes 're' (prefix meaning 'again').
'one had' says to put letters next to each other.
'left' becomes 'al' (I've seen this in other clues).
're'+'al'='REAL'
'alone it's' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for real that I've seen before include "Genuine - old Spanish coin" , "Actual, concrete" , "Actual and tangible" , "The old coin actually exists" , "Not imaginary, genuine" .)