A possible Castilian naval officer keeping fit, mostly (5)
I believe the answer is:
pablo
'fit mostly' is the definition.
The definition suggests a singular noun which matches the answer.
'a possible castilian naval officer keeping' is the wordplay.
'possible castilian' becomes 'bl' (I can't justify this - if you can you should believe this answer much more).
'naval officer' becomes 'po' (abbreviation for petty officer).
'keeping' is an insertion indicator (some letters kept inside).
'a'+'bl'='abl'
'abl' placed inside 'po' is 'PABLO'.
Can you help me to learn more?
(Other definitions for pablo that I've seen before include "Foreign fellow" , "Paul in Spain" , "Spanish boy's name" , "Picasso's name" , "and 13 Down: Spanish artist" .)