Type of wave to a boy getting it back (5)
I believe the answer is:
tidal
'type of wave' is the definition.
(I've seen this in another clue)
'boy getting it back' is the wordplay.
'boy' becomes 'lad' (informal term for boy).
'getting' says to put letters next to each other.
'back' shows that the letters should be reversed in order.
'lad'+'it'='ladit'
'ladit' back-to-front is 'TIDAL'.
'to a' acts as a link.
(Other definitions for tidal that I've seen before include "To do with movement of seas" , "Relating to sea rise/fall" , "Ebbing and flooding" , "Pertaining to rise and fall of the sea" , "Relating to the flow of the sea" .)