Manual worker (5)
(Other definitions for navvy that I've seen before include "Construction worker" , "Labourer, originally digging the canals" , "Canal labourer" , "Construction labourer" , "Labourer working on canals, roads, railways etc?" .)
'manual worker' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for labourer that I've seen before include "Unskilled worker" , "Workman" , "Navvy" , "Hercules, perhaps?" , "One is worthy of his hire, according to the Bible" .)