Barriers the reverse of substantial, weak and small (9)

'barriers' is the definition.
(taffrail is a kind of barrier)

'reverse of substantial weak and small' is the wordplay.
'reverse of' is a reversal indicator.
'substantial' becomes 'fat' (fat can mean large or substantial).
'weak' becomes 'frail' (similar in meaning).
'and' is a charade indicator (letters next to each other).
'small' becomes 's' (abbreviation - e.g. clothes size).
'fat' reversed gives 'taf'.
'taf'+'frail'+'s'='TAFFRAILS'

'the' acts as a link.