Run to join forces and gain strength (5)
I believe the answer is:
rally
'gain strength' is the definition.
(thesaurus)
'run to join forces' is the wordplay.
'run' becomes 'r' (cricket abbreviation).
'to join forces' becomes 'ally' (I've seen this before).
'r'+'ally'='RALLY'
'and' is the link.
(Other definitions for rally that I've seen before include "Call to arms - series of strokes" , "Get-together" , "Mass meeting -- revive" , "Recover health" , "exchange over the Net?" .)