Trend bound to be picked up by radio (4)
I believe the answer is:
tide
'trend' is the definition.
(I've seen this in another clue)
'bound to be picked up by radio' is the wordplay.
'bound' says to hollow out the word (remove centre letters) (I've seen 'bounds of' mean this (the letters which form the boundaries of the word)).
'to be picked up' becomes 'tidied' (I've seen this in another clue).
'by radio' indicates a 'sounds like' (homophone) clue (how the word might sound on the radio).
'tidied' with its middle removed is 'tied'.
'tied' is a homophone of 'TIDE'.
(Other definitions for tide that I've seen before include "Moon-driven sea movement" , "Ebber and flower?" , "Regular movement" , "Rising and falling of the sea" , "Powerful trend of events" .)