One's invested in phone for rally (8)
I believe the answer is:
mobilise
'rally' is the definition.
(I've seen this before)
'one's invested in phone' is the wordplay.
'one's' becomes 'I's' (Roman numeral).
'invested in' indicates putting letters inside.
'phone' becomes 'mobile' (mobile phone).
'is' going inside 'mobile' is 'MOBILISE'.
'for' is the link.
(Other definitions for mobilise that I've seen before include "Prepare for war" , "Get ready for war" , "prepare for active service" , "rally" , "Make ready for action, troops for war perhaps" .)