Look for some easy anagrams to start.
or ...5a if you google Aldershot born novelist ...he's 1st up...then for the letter to lose...look at the wordplay...
to strengthen = man
with ce Inside......
1d Clare refers to a nun ...so you need a synonym
2d cardinal refers to a number
4d the wordplay is an anagram of serialtobe...try adding random extra letter (it only occurs the once) to get a synonym for simple
I got quite a few answers by working backwards from the definitions...look for a synonym then look what the wordplay might be if you lose a letter(s) from the synonym
17a the underwear are for men ...the usual 5, 6 ...then lose a letter