Mole was worried about each pair of lines
I believe the answer is:
seawall
'mole' is the definition.
(synonyms)
'was worried about each pair of lines' is the wordplay.
'worried' is an anagram indicator.
'about' means one lot of letters goes inside another.
'each' becomes 'ea'.
'pair of' shows that letters should be duplicated (I've seen 'pair' mean this).
'lines' becomes 'l' (this could be a standard abbreviation which I don't know about).
'was' anagrammed gives 'swa'.
'swa' enclosing 'ea' is 'seawa'.
'l' duplicated is 'll'.
'seawa'+'ll'='SEAWALL'
Can you help me to learn more?
(Another definition for seawall that I've seen is " sea defence".)