Sunny on a sailing ship and hot outside (6)
I believe the answer is:
bright
'sunny' is the definition.
(similar in meaning)
'sailing ship and hot outside' is the wordplay.
'sailing ship' becomes 'brig' (brig is a kind of sailing ship).
'and' means one lot of letters go next to another.
'outside' suggests removing the centre.
'hot' with its middle removed is 'ht'.
'brig'+'ht'='BRIGHT'
'on a' is the link.
(Other definitions for bright that I've seen before include "Shiny" , "Jolly" , "7 [CLEVER]" , "Clever - shining - sunny" , "supporter of free trade" .)