Like most cryptic indicators, grammar and sense dictate where they should appear in relation to their fodder (the part of the clue the indicator works on).For example, "[fodder] sounds like [definition]" clearly indicates that a homophone of [fodder] is a synonym of [definition] (and therefore the solution), whereas "[definition] sounds like [fodder]" does not.All that is to say, it is important to parse the clue and ask yourself what it's telling you to do literally. Below is a link to an extensive list of anagram indicators.
"Amplified groan on the radio (5)" -> 'on the radio' is a homophone indicator. We're looking for a synonym of 'amplified' that is also a homophone of 'groan' -> GROWN.