Get on (5)
'get on' is the definition.
(eg boarding a train)
This is the entire clue.
(Other definitions for board that I've seen before include "Get on - governing body" , "Food - table - directors" , "See 13 across" , "Strip of timber" , "Collection of directors" .)
(Other definitions for mount that I've seen before include "Fix onto a backing" , "Climb - Everest, for example" , "Get on - horse" , "seat" , "The height" .)
'get on' is the definition.
(synonyms)
This is all the clue.
(Other definitions for age that I've seen before include "Length of life" , "Years since birth" , "Time in years" , "Period of time" , "Grow older" .)