Moor steamship, possibly for alteration (13)
I believe the answer is:
metamorphosis
'alteration' is the definition.
(metamorphosis is a kind of alteration)
'moor steamship possibly' is the wordplay.
'possibly' is an anagram indicator.
'moor'+'steamship'='moorsteamship'
'moorsteamship' is an anagram of 'METAMORPHOSIS'.
'for' is the link.
(Other definitions for metamorphosis that I've seen before include "Total change in character" , "Complete change of appearance" , "Total change in form" , "Transmutation" , "Process of transformation" .)