Shout at youngster climbing platform in the sea (3,3)
I believe the answer is:
oil rig
'platform in the sea' is the definition.
(an oil rig is a platform standing in the sea)
'shout at youngster climbing' is the wordplay.
'shout' becomes 'oi' ('oi' might be shouted to get someone's attention).
'at' is a charade indicator (letters next to each other).
'youngster' becomes 'girl'.
'climbing' is a reversal indicator (in a down clue, letters climb up).
'girl' reversed gives 'lrig'.
'oi'+'lrig'='OIL-RIG'
(Other definitions for oil rig that I've seen before include "Well drilling plant" , "What may be boring" , "platform in the main" , "Structure for drilling for viscous liquid" , "Fuel well" .)