Guide personnel to river (6)
I believe the answer is:
mentor
'guide' is the definition.
(thesaurus)
'personnel to river' is the wordplay.
'personnel' becomes 'men' (men is a kind of personnel).
'river' becomes 'r'.
'men'+'to'+'r'='MENTOR'
(Other definitions for mentor that I've seen before include "A trusted adviser or teacher" , "One who steers" , "Truster advisor" , "One guiding" , "A wise and experienced teacher" .)