Jogamel. the answers are entered in the order of the clue.
Neighbours... leads to a six-letter word meaning 'reviewer'. 'A different side of' is the usual instruction to change a letter. The answer is a word meaning 'capable of being pronounced with full-word stress'. It's often relating to French words, hence 'neighbours'.
The rest of the clue is for the other side. There's a letter not given by wordplay. You need a three-letter 'us' after a two-letter abbreviation for 'posties'. The 'mail' isn't letters or parcels!