Criminals held back by Thai family (5)
I believe the answer is:
mafia
'criminals' is the definition.
(I've seen this before)
'held back by thai family' is the wordplay.
'back by' says the letters should be written in reverse.
'held' indicates a hidden word.
'aifam' is hidden within 'thai family'.
'aifam' written backwards gives 'MAFIA'.
(Other definitions for mafia that I've seen before include "Criminal group" , "Sicilian criminals" , "Criminal society" , "Secret criminal organisation" , "Sicilian one" .)