Periodic ocean lad with a rear admiral (8)
I believe the answer is:
seasonal
'periodic' is the definition.
(thesaurus)
'ocean lad with a rear admiral' is the wordplay.
'ocean' becomes 'sea' (synonyms).
'lad' becomes 'son' (lad can informally mean a young boy).
'with' says to put letters next to each other.
'a rear admiral' becomes 'al' (I can't justify this - if you can you should give a lot more credence to this answer).
'sea'+'son'+'al'='SEASONAL'
Can you help me to learn more?
(Other definitions for seasonal that I've seen before include "depending on the month" , "Occurring at a particular time of the year" , "Fluctuating according to the time of year" , "specifically for Christmas period, say" , "Occurring at certain times of the year" .)