Praise from partner once was effective, mostly (5)
I believe the answer is:
extol
'praise' is the definition.
(I know that extol is a more specific form of the action praise)
'partner once was effective mostly' is the wordplay.
'partner once' becomes 'ex' (an ex-partner).
'was effective' becomes 'told' (I've seen this in another clue).
'mostly' means to remove the last letter (most but not all of the word).
'told' with its final letter removed is 'tol'.
'ex'+'tol'='EXTOL'
'from' is the link.
(Other definitions for extol that I've seen before include "Exalt" , "Sing the praises of" , "Praise - honour" , "'Praise, glorify (5)'" , "Proclaim" .)