Hi Nicola,
12a and 11d are both double definitions. For 12a think how difficult children might be described.
The first 5 letters of 4a are a homophone for a word that means 'brings on cargo' and the last 3 letters are another word for bitumen. The definition is something that assists sailors when out at sea (not a person, as I originally thought)
Can anyone help me parse 6d? I largely biffed it, after realising that the second word was part of a rhyming pair, but have no idea what El Salvador has to do with it, or anything else in the clue really!