Multiple trip to mid-Solent (6)
I believe the answer is:
triple
'multiple' is the definition.
(similar in meaning)
'trip to mid-solent' is the wordplay.
'to mid' says to take the centre.
The middle of 'solent' is 'le'.
'trip'+'le'='TRIPLE'
(Other definitions for triple that I've seen before include "involving several parties" , "make score sixty?" , "make much longer" , "Consisting of three" , "Multiply by three" .)