Perfect backside a lad is concealing (5)
I believe the answer is:
ideal
'perfect' is the definition.
(similar in meaning)
'backside a lad is concealing' is the wordplay.
'is concealing' indicates the answer is hidden within the clue.
'IDEAL' is hidden within 'backside a lad'.
(Other definitions for ideal that I've seen before include "Model of excellence (5)." , "Standard to aim for" , "Perfectly suited" , "without a flaw" , "To which one aspires" .)