Small sailing boat, first of yachts, partly designed (7)
I believe the answer is:
sketchy
'partly designed' is the definition.
The answer and definition are not the same part of speech. However, past participle verbs and adjectives occasionally mean the same thing.
'small sailing boat first of yachts' is the wordplay.
'small sailing boat' becomes 'sketch' (I've seen this before).
'first of yachts' becomes 'y' (1st letter of 'yachts').
'sketch'+'y'='SKETCHY'
Can you help me to learn more?
(Other definitions for sketchy that I've seen before include "Incomplete; rough" , "Vague" , "Not thorough or detailed" , "Cursory" , "Barely outlined" .)