Phone feature to increase in value (8)
I believe the answer is:
appraise
'value' is the definition.
(synonyms)
'phone feature to increase' is the wordplay.
'phone feature' becomes 'app' (I've seen this in another clue).
'to increase' becomes 'raise' (raise can mean to increase or put up).
'app'+'raise'='APPRAISE'
'in' acts as a link.
(Other definitions for appraise that I've seen before include "Assess, set a price on" , "Assess or evaluate" , "survey?" , "Estimate or assess the value of" , "Assess (a thing's) quality" .)