A plan for gaining weight (9)
I believe the answer is:
programme
'a plan' is the definition.
(I know that programme is a type of schedule)
'for gaining weight' is the wordplay.
'for' becomes 'pro' (as in being 'pro' something).
'gaining' means one lot of letters go next to another.
'weight' becomes 'gramme' (I have seen 'Small weight ' mean 'gramme' so perhaps 'weight' could also mean 'gramme').
'pro'+'gramme'='PROGRAMME'
(Other definitions for programme that I've seen before include "Syllabus" , "Schedule of proceedings" , "Radio or TV show" , "Curriculum" , "Schedule of events" .)