Verse with a line to alter (4)
I believe the answer is:
vary
'alter' is the definition.
(synonyms)
'verse with a line' is the wordplay.
'verse' becomes 'v'.
'with' means one lot of letters go next to another.
'a line' becomes 'ary' (I can't justify this - if you can you should give a lot more credence to this answer).
'v'+'ary'='VARY'
'to' is the link.
Can you help me to learn more?
(Other definitions for vary that I've seen before include "Diversify" , "Make or become different" , "Change, diverge, deviate" , "Change according to circumstances" , "Change form or state" .)